How to Thicken Frosting
To thicken frosting, sprinkle an additional 1 to 2 tablespoons of powdered sugar into the mixture and combine thoroughly. For frosting made on the stovetop, sprinkle 1 teaspoon of flour into the frosting and stir well over low heat to incorporate the flour. If you're worried about making the frosting too sweet, add 1 ounce of softened cream cheese to the frosting to thicken it without adding more sugar. For chocolate-based frostings, you can also add 1 to 2 teaspoons of extra cocoa powder for thickening.
Add more powdered sugar. Most frostings contain powdered sugar, and the most common way to thicken a runny frosting is by gradually mixing in a bit more powdered sugar to offset the liquid ingredients. Only add the sugar a little at a time. Sprinkle an additional 1 to 2 Tbsp (15 to 30 ml) into the frosting and mix well. If you add too much at once, the frosting will become too sweet and may become too thick. If this happens, you will need to add more liquid to balance it out, and the consistency problem may only continue from there. Powdered sugar contains cornstarch. Starches absorb liquid, and the cornstarch present in powdered sugar prevents the powdered sugar from caking. Mix in a bit of meringue powder alongside the powdered sugar. To cut down on the sweetness of powdered sugar, you can add meringue powder, as well. It will thicken your frosting without contributing any sweetness of its own. Typically, if you are adding more than 1/2 cup (125 ml) of powdered sugar to thicken a runny royal icing recipe, you should also add 1 to 2 tsp (5 to 10 ml) of meringue powder. This is especially true if the recipe already contains meringue powder. Meringue powder is made from dried egg whites, sugar, and gum. The sugar can absorb some of the liquid and the gum is a natural thickener. If used in excess, however, you might end up with a frosting that is too heavy or stringy. Use tapioca, cornstarch, or arrowroot starch. Dry starches like these absorb liquid and are a great way to thicken frosting without altering the taste of the frosting. Add roughly 1 Tbsp (15 ml) of starch to your frosting. Heat the frosting over low heat, if it can withstand higher temperatures, while stirring the starch in. Remove from the stove as soon as the frosting begins to thicken. Cornstarch is probably the most popular starch to use in frostings since it has low gloss, little flavor, and works well with dairy. It congeals when exposed to low temperatures, though, so it is not ideal for use with frostings that need to remain refrigerated. It works best for frostings that are cooked on the stovetop slightly before cooling to room temperature. Arrowroot produces a high gloss and works better with acidic liquids. It tends to become slimy when added to dairy, but if your frosting contains dairy products with a higher acidity, like sour cream or buttermilk, arrowroot is a good choice. It also thickens at a fairly low temperature, making it suitable for frostings that will need to remain cool. Tapioca also has a high gloss, but it stands up well to cold temperatures and thickens at a fairly low temperature, as well, making it a better option if you have a frosting that will need to remain cold. Only use flour for cooked frostings. If you are making warm frosting on your stovetop, you can try thickening a runny frosting with a bit of flour. Do not use flour in cold frosting recipes, however. Flour has a distinctive taste when eaten raw, and the only way to get rid of this taste is by cooking it out. As such, you can use flour for cooked frostings, assuming the frosting is still undergoing the heating process, but it is not a suitable thickener for cool frostings that require no cooking. Flour will not reach its maximum thickening capability if it is not heated. Sprinkle 1 tsp to 1 Tbsp (5 ml to 15 ml) of flour into the frosting and stir over low heat on the stove until it begins to thicken. Remove the frosting from the heat as soon as it begins to thicken. Cooking flour in for too long can cause the frosting to become thin and runny again. Try softened cream cheese. If your frosting is too sweet as well as being too runny, you can try adding cream cheese to it as a way to thicken the consistency and add a contrasting tanginess to the existing sweetness. Add about 1 oz (30 ml) of cream cheese to your frosting and blend it in well. This works best with frostings that already contain cream cheese or those that could benefit from less sweetness. Increase the amount of cocoa powder, when applicable. While this is not an option for vanilla, cream cheese, or other non-chocolate frostings, the addition of more cocoa powder is worth trying if you are making a chocolate frosting. Add more cocoa powder into the frosting 1 to 2 tsp (5 to 10 ml) at a time. Otherwise, you may end up with a frosting that is too thick or too potent. Since cocoa powder is also fairly bitter on its own, too much can also cause the frosting to become bitter. Cocoa powder is a starch thickener, as well, but does not need to be cooked in order to thicken liquid. As a result, cocoa powder is a better addition than melted chocolate when you are trying to thicken your frosting. Unsweetened cocoa has more thickening power than bittersweet chocolate. The former contains more starches than the latter. Mix in more butter or shortening. For frostings that use butter or shortening, using more of either ingredient can help thicken a frosting that is a little too runny. Only add 1 Tbsp (15 ml) or so at a time to prevent distorting the taste and consistency too drastically. Note that adding butter to the frosting may not seem to help immediately. As the frosting heats up from being mixed, the butter will melt, making the consistency thinner. Refrigerate butter-based frostings to get an accurate picture of how thick the frosting really is. Consider mixing in a dash of heavy cream. For frostings that can stand to be beaten and chilled, heavy cream is another way you can thicken the consistency without making the taste too sweet. Cream will provide a thick, rich texture. This works best if you plan on heating or whipping your frosting after you add the cream. Heating cream causes it to reduce down, becoming thicker as a result. Whipping cream causes it to fluff up, which also makes it thicker and causes the frosting to become lighter in texture. Only add about 1/4 cup (60 ml) heavy cream to the frosting. Too much will only make the frosting thinner. Refrigerate the frosting. Some frostings seem thin simply because they need time to set. If this is the case, chilling the frosting in your refrigerator for 30 to 90 minutes will usually do the trick. This trick generally works best for frostings that have been cooked on the stove. Place the frosting in your refrigerator to let it set. As the temperature drops, the consistency will usually thicken, as well. This can also work if your frosting is butter-based or if it contains whipped cream. These ingredients are heat sensitive, so the longer they are exposed to room temperature, the thinner the frosting will seem. Placing the frosting back in the refrigerator will cause it to thicken back up. Cook hot frosting for more time. If a stovetop frosting still seems a little too liquidy, you might be able to reduce some of that liquid by extending the cooking time slightly. Note, however, that you should exercise caution while trying this technique. If you cook a frosting too long, it may burn or break down. Stir the frosting frequently as it cooks, and if it does not appear to start thickening after a few additional minutes, remove it from the heat and try a different tactic. Follow the instructions closely. Frosting might seem simple, but making even a minor change to the ingredients can result in an unexpected change to the consistency. Unsweetened chocolate will make your frosting thicker than bittersweet or milk chocolate. Cocoa solids have starches in them, and unsweetened chocolate has a higher amount of cocoa solids in it while sweeter chocolates have more sugar and less starch. As a result, if a recipe calls for unsweetened chocolate and you use semisweet, your frosting may turn out thinner than expected. Cream cheese and milk are other examples. Generally, there will not be much difference between a frosting made with 2% milk and whole milk, but if a recipe calls for half-and-half or heavy cream, do not substitute milk in its place. Similarly, low-fat cream cheese has been known to produce thinner frostings when a recipe calls for standard cream cheese. Mix the ingredients together in the best possible sequence. Typically, the fully liquid ingredients in a frosting are added last. Following this procedure could help you to prevent runny frosting before it happens. If the sugar and butter (or shortening) are mixed first, add other liquids like water and milk afterward. In this instance, these ingredients are only added to make the frosting easier to whip and spread, and should be added cautiously to prevent the frosting from becoming runny. Add ingredients slowly and in small amounts. One of the biggest reasons why people often end up with consistency problems while making frosting is simply that a little too much of one ingredient gets added, throwing the entire recipe off. Both the liquids and powdered sugar need to be added slowly and in small amounts. If a frosting ends up being way too thick at first, the need to thin it by adding more liquid can result in the addition of too much liquid, which will make the frosting too thin. 
Add more powdered sugar. Mix in a bit of meringue powder alongside the powdered sugar. Use tapioca, cornstarch, or arrowroot starch. Only use flour for cooked frostings. Try softened cream cheese. Increase the amount of cocoa powder, when applicable. Mix in more butter or shortening. Consider mixing in a dash of heavy cream. Refrigerate the frosting. Cook hot frosting for more time. Follow the instructions closely. Mix the ingredients together in the best possible sequence. Add ingredients slowly and in small amounts. 