01 - Preheat oven to 350°F and 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 together 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 the batter is evenly colored.
06 - Alternate adding dry ingredients and buttermilk to the wet mixture, beginning and ending with dry ingredients. Mix until just combined, taking care not to overmix.
07 - Drop 24 evenly sized mounds of batter onto the prepared baking sheets using a cookie scoop or tablespoon, spacing 2 inches apart.
08 - Bake for 10-12 minutes or until the cakes spring back when lightly touched. Cool completely on a wire rack.
09 - Beat butter 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 store in an airtight container in the refrigerator for up to 3 days.