Preheat oven to 350°F. Lightly spray Whoopie Pie Pan with nonstick cooking spray.
2In large bowl, beat butter and sugar with electric mixer on medium speed until light and fluffy, about 3 minutes. Add egg and vanilla; beat until well combined.
3In 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 to mix 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; spread batter to the edges. Cavity should be about 2/3 full.
4Bake for 8-10 minutes or until the top of the cake springs back when touched. Allow cakes to cool in pan 8 minutes. Remove from pan and cool completely before filling. To assemble pies spread filling on one cake and sandwich with another cake.
In large bowl, beat butter with electric mixer until creamy. Add sugar and salt to the butter; mix well. Add marshmallow crème and vanilla; blend well. Use immediately to fill whoopie pies or refrigerate covered up to 1 week. Bring back to room temperature and lightly beat before using.