01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt until evenly blended.
03 - In a large bowl, beat softened butter and granulated sugar using an electric mixer until light and fluffy. Add egg and continue beating until fully incorporated.
04 - Add buttermilk and vanilla extract to the creamed mixture. Gradually mix in the dry ingredients until batter is smooth and homogenous.
05 - Using a spoon or cookie scoop, drop 24 evenly sized rounds of batter onto the prepared baking sheets, spacing each about 2 inches apart.
06 - Bake for 10 to 12 minutes until cakes are set and surface springs back when gently touched. Transfer to a cooling rack and let cool completely.
07 - In a clean bowl, beat softened butter until creamy. Add powdered sugar, marshmallow creme, vanilla extract, and a pinch of salt. Mix until filling is light and fluffy.
08 - Divide the filling evenly into three bowls. Leave one batch white, color another with red food coloring, and tint the third with blue food coloring.
09 - Pipe or spoon a thick layer of filling onto the flat side of half the cooled cakes. Cap with remaining cakes to form sandwiches.
10 - Roll the edges in red, white, and blue sprinkles or use piping bags to add decorative frosting to the tops.