01 - Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
03 - In a large bowl, beat unsalted butter and granulated sugar until light and fluffy. Incorporate egg and mix thoroughly.
04 - Add buttermilk and vanilla extract to the creamed mixture. Gradually incorporate dry ingredients until a smooth batter forms.
05 - Drop 24 equal rounds of batter onto prepared baking sheets, spacing 2 inches apart.
06 - Bake 10–12 minutes or until cakes are set and lightly spring back when touched. Transfer to a cooling rack and allow to cool completely.
07 - In a clean bowl, beat unsalted butter until creamy. Add powdered sugar, marshmallow creme, vanilla extract, and a pinch of salt. Whip mixture until airy and light.
08 - Divide filling evenly among three bowls. Leave one uncolored, tint one with red food coloring, and tint the last with blue food coloring.
09 - Pipe or spoon filling onto the flat side of half the cakes. Top with remaining cakes to form sandwiches.
10 - Roll filled edges in patriotic sprinkles or pipe colored frosting on top as desired for celebratory presentation.