01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Whisk together flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar in a large bowl until light and fluffy, approximately 3 minutes.
04 - Add egg and vanilla extract to the butter mixture, beating until fully incorporated.
05 - Mix in red food coloring until batter is evenly colored throughout.
06 - Alternate adding dry ingredients and buttermilk to the wet mixture, starting and ending with dry ingredients. Mix until just combined to avoid overmixing.
07 - Drop 24 evenly sized mounds of batter onto prepared baking sheets using a cookie scoop or tablespoon, leaving 2 inches of space between each.
08 - Bake for 10 to 12 minutes, or until cakes spring back when lightly touched. Cool completely on a wire rack.
09 - Beat butter in a bowl until creamy. Add powdered sugar and beat until fluffy.
10 - Add marshmallow creme, vanilla, and salt. Beat until smooth and airy.
11 - Spread or pipe a generous tablespoon of marshmallow filling onto the flat side of half the cakes. Top with remaining cakes to form sandwiches.
12 - Serve immediately or refrigerate in an airtight container for up to 3 days.