You have to be careful on some flavorings, especially banana. Some flavorings are really disgusting, as banana is a hard flavor to get right.
Most banana flavors are super sweet and taste like fake, plastic bananas to me. I have had a couple that tasted ok, but they were still fake tasting.
You might want to think about switching......make a banana cupcake with mashed bananas and peanut butter frosting.
If you want banana frosting, then I would use mashed bananas, not banana flavoring. Unless you happen to find a really good flavoring you like.
Or
You could make a peanut butter cupcake, with caramel frosting, and put a banana pudding filling in it.
If you make the banana buttercream, put it in an airtight container, put some plastic wrap directly on the frosting in the container, then close it airtight. Put in the fridge overnight.
Take it out of the fridge about 30 minutes before you want to use it, and lightly stir it up before using.
Letting it sit overnight will help the banana permeate into the frosting better.