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.
Make the cupcakes: Sift the flour, cornstarch, baking powder, and salt together in a large bowl. Set aside. Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy - about 1 minute.