01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together all-purpose flour, unsweetened cocoa powder, baking soda, and salt until evenly combined.
03 - In a large bowl, beat unsalted butter and granulated sugar with an electric mixer until light and fluffy. Add the egg and mix thoroughly.
04 - Add buttermilk and vanilla extract to the butter mixture. Gradually incorporate the dry mixture, blending until batter is smooth.
05 - Spoon or scoop 24 equal portions of batter onto prepared trays, spacing 2 inches apart. Bake for 10 to 12 minutes, or until the cakes are set and spring back when touched. Let cool completely on a wire rack.
06 - Beat softened butter until creamy. Add powdered sugar, marshmallow creme, vanilla extract, and a pinch of salt. Beat until the mixture is light and fluffy.
07 - Divide the filling evenly into three bowls. Leave one batch plain, tint one batch red, and tint the remaining batch blue with food coloring.
08 - Pipe or spread an even layer of filling on the flat side of half of the cakes. Top with the remaining cakes to create sandwiches.
09 - Roll the edges of each whoopie pie in patriotic sprinkles, or pipe additional colored frosting on top if desired.