01 - Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt until thoroughly combined.
03 - In a large bowl, beat unsalted butter with granulated sugar until the mixture is light and fluffy. Incorporate the egg until well blended.
04 - Add buttermilk and vanilla extract to the creamed mixture. Gradually incorporate the dry ingredients, mixing until a smooth batter forms.
05 - Using a spoon or cookie scoop, drop 24 evenly sized portions of batter onto the prepared sheets, spacing each 2 inches apart.
06 - Bake for 10 to 12 minutes until cakes are set and spring back when lightly pressed. Allow to cool completely on a rack.
07 - Beat softened butter until creamy. Gradually add powdered sugar, marshmallow creme, vanilla extract, and a pinch of salt. Beat until light and fluffy.
08 - Divide the filling into three bowls. Leave one bowl plain, tint another with red food coloring, and the last bowl with blue food coloring.
09 - Apply a layer of filling to the flat side of 12 cakes using a piping bag or spoon. Top each with a remaining cake to form sandwiches.
10 - Roll the edges of assembled pies in red, white, and blue sprinkles or pipe colored filling on top for festive decoration.