Preheat oven to 350 degrees. Lightly spray Whoopie Pie Pan with nonstick spray.
In 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 we'll 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 mixer and add buttermilk. Continue to mix on low 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 edges. Cavity should be 2/3 full.
Bake 8-10 minutes or until the top of the cake springs back when touched. Allow cakes to cool in pan 8 minutes. Cool completely before filling. To assemble pies spread filing on one cake and sandwich with another cake.
Whoopie Pie Filing: In large bowl, beat butter with electric mixer until creamy. Add sugar and salt to the butter, mix we'll. add marshmallow cream and vanilla; blend well. Immediately fill Whoopie Pies. Can refrigerate covered for up to 1 week. Bring back to room temperature & beat lightly before using.