01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt until thoroughly mixed.
03 - In a large bowl, beat unsalted butter and granulated sugar with an electric mixer until the mixture is light and fluffy. Add the egg and mix until blended.
04 - Pour in buttermilk and vanilla extract, then gradually mix in the dry ingredients until a smooth batter forms.
05 - Drop 24 evenly sized rounds of batter onto the prepared baking sheets, leaving 2 inches of space between each. Bake for 10 to 12 minutes, or until set and cakes spring back when gently touched. Let cool completely on a cooling rack.
06 - Beat softened butter until creamy. Add powdered sugar, marshmallow creme, vanilla extract, and salt. Continue beating until the filling is light and fluffy.
07 - Divide filling evenly into three bowls. Leave one uncolored, tint the second with red food coloring, and the third with blue food coloring to achieve vibrant shades.
08 - Pipe or spoon a portion of colored filling onto the flat side of half of the cakes. Top with the remaining cakes to form sandwiches.
09 - Roll the edges in patriotic sprinkles or pipe extra frosting on top for decorative flair.