I made these pies for my Activity Days girls this afternoon, and they were a big hit. They are super easy, and, for once in my life, mine were just as pretty as the ones in the pictures. :) I used my cookie scoop to guarantee all the cookies were the same size, and it was perfect.
I used a different filling, however, since I didn't have enough peanut butter to make the filling in the recipe.
Here's the filling I used:
Peanut Butter Filling
1 cube unsalted butter
1 8-oz package low-fat cream cheese
1/2 cup peanut butter
3 tbsp. half-and-half
1/2 tsp. salt
about 2 cups powdered sugar
Beat peanut butter, butter, and cream cheese together until perfectly smooth and uniform. Add half-and-half and salt, then beat in powdered sugar until fluffy. Adjust consistency with more powdered sugar or more half-and-half. Scrape bowl into gallon-sized ziploc bag, squeeze out excess air, snip off the corner, and pipe onto one of the two whoopie pie cookies, topping with another cookie.
This filling would be delicious between two layers of chocolate cake as well. It's fantastic.