Directions
1
Cakes: Preheat oven to 350˚F. Whisk together flour, cocoa powder, baking soda and salt; set aside.
2
Using electric mixer, beat sugar and butter until light and fluffy; beat in egg. Beat in vanilla. Reduce speed to low; mix in flour mixture alternately with buttermilk, beginning and ending with flour mixture.
3
Drop mounds of heaping 2 tbsp batter onto 2 parchment paper–lined baking sheets, about 2 inches apart (you should have 24 whoopie pie cake halves).
4
Bake for 10 to 13 minutes or until tester comes out clean when inserted into center of cakes. Transfer to rack; let cool completely.
5
For the filling: Beat together marshmallow fluff, icing sugar, butter and vanilla until light and fluffy. Reserve 12 M&M's. Fold remaining M&M's into marshmallow mixture.Tip: Buttermilk substitute: Add 2 tsp white vinegar to measuring cup; pour in enough milk to make 1/3 cup. Let stand for 5 minutes before using. Substitute prepared cream cheese frosting for marshmallow filling if desired.
6
Spread 11/2 tsp icing over flat side of 12 whoopie pie cake halves. Cap with remaining cakes.
7
Dollop remaining marshmallow fluff over tops of whoopie pies; decorate each whoopie pie with 1 M&M's.
