As a rule, a CTE will NEVER improve performance. A CTE is essentially a disposable view. There are no additional statistics stored, no indexes, etc. It functions as a shorthand for a subquery. In my opinion they can be EASILY overused (I see a lot of overuse in code in my job).
Check the cupcakes at the minimum baking time. If the recommended baking time is 18-22 minutes, check them at 17 or 18 minutes (just a quick peek, make sure not to let too much heat out of the oven). If the tops are rounded and no long appear wet, they may be done.
Allow the cupcakes to cool completely on a baking rack after baking. Place the cupcakes in an airtight plastic container and store in the refrigerator for three or four days. Before frosting, remove the cupcakes from the fridge and place them in a dry area on a counter or table to bring them to room temperature.