• Preheat oven to 350 degrees. Lightly spray Whoopie Pie pan with non-stick spray.
• In large bowl, beat butter and sugar with electric mixer on medium until light and fluffy, about 3 minutes. Add egg and vanilla; beat until well combined.
• In another bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Add half the dry ingredients into the butter mixture and mix on low speed.
• When dry ingredients are almost incorporated, stop the mixer and add the buttermilk.
• Continue mixing on low speed until all ingredients are almost incorporated. Stop the mixer and add the remaining dry ingredients. Mix on low until just combined.
• Spoon 2 tablespoons of batter into each cavity. Make sure batter is spread to the edges. Cavity should be about ⅔ full.
• Bake for 8-10 minutes or until the top of the cake springs back when touched.
• Allow cakes to cook in pan for 5 minutes. Remove from pan and cool completely before filling.
• To assemble pies, spread about 1 tablespoon filling on one cake and sandwich with another cake.

Filling

• In large bowl, beat butter with mixer until creamy.
• Add sugar and salt to the butter; mix well.
• Add marshmallow creme and vanilla; blend well.
• Add food coloring if desired.
• Use immediately to fill whoopie pies or refrigerate up to 1 week. Bring back to room temperature and beat lightly before using.