01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Whisk flour, cocoa powder, baking soda, and salt in a medium bowl.
03 - In a large bowl, beat softened butter and sugar until light and fluffy. Add egg and combine thoroughly.
04 - Add buttermilk and vanilla to the wet mixture, then gradually fold in the dry ingredients. Mix until batter is smooth and uniform.
05 - Drop 24 even rounds of batter onto the prepared baking sheets, spacing each about 2 inches apart. Bake for 10–12 minutes, until cakes spring back lightly when touched. Cool completely on a rack.
06 - Beat softened butter until creamy. Add powdered sugar, marshmallow creme, vanilla, and pinch of salt. Mix until filling is light and fluffy.
07 - Divide the filling evenly among three bowls. Leave one bowl plain, tint the second with red food coloring, and the third with blue food coloring.
08 - Using a spoon or piping bag, spread or pipe a portion of filling onto the flat side of half the cakes. Top with remaining cakes to make sandwiches.
09 - Roll the edges of each assembled whoopie pie in patriotic sprinkles for a festive finish. Optionally, pipe colored frosting on top for additional decoration.