A simple recipe for whoopie pie filling comes from Epicurious.com. It features combining butter, confectioners sugar, marshmallow and vanilla and is then beaten with an electric mixer until the filling is smooth.
To start, add 1 stick of softened unsalted butter, 1 1/4 cups of confectioners sugar, 2 cups of marshmallow cream and 1 teaspoon of vanilla into a large mixing bowl. Whip with the electric mixer for about 3 minutes.
When making the whoopie pie cakes, preheat the oven to 350 F. Combine 2 cups of all-purpose flour, 1/2 cup of cocoa powder, 1 1/4 teaspoons of baking soda, and 1 teaspoon of salt in a large mixing bowl. In a separate bowl, mix together 1 cup of buttermilk and 1 teaspoon of vanilla.
In a third mixing bowl, beat together 1 stick of softened unsalted butter and 1 cup of brown sugar with an electric mixer at medium-high speed. After the mixture is pale and fluffy, add one egg, and mix well. Reduce the speed to low, and mix in flour and buttermilk mixtures slowly in batches, making sure to both begin and end with the flour.
Spoon the batter into 1/4-cup mounds on two greased baking sheets. Place the sheets in the upper part of the oven, and bake for 11 to 13 minutes, switching the positions of the sheets halfway through baking. Allow the cakes to cool completely on a rack prior to adding filling.