Cakes: Preheat oven to 350˚F. Whisk together flour, cocoa powder, baking soda and salt; set aside.
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.
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).
Bake for 10 to 13 minutes or until tester comes out clean when inserted into center of cakes. Transfer to rack; let cool completely.
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.
Spread 11/2 tsp icing over flat side of 12 whoopie pie cake halves. Cap with remaining cakes.
Dollop remaining marshmallow fluff over tops of whoopie pies; decorate each whoopie pie with 1 M&M's.