01 - Preheat the 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 with an electric mixer until light and fluffy, about 2–3 minutes.
04 - Add egg and vanilla extract; beat until well combined. Mix in red food coloring until evenly distributed.
05 - On low speed, alternately add the flour mixture and buttermilk in three additions, starting and ending with the flour. Mix until just combined.
06 - Using a tablespoon or cookie scoop, drop 24 evenly-sized mounds of batter onto the prepared sheets, spacing 2 inches apart.
07 - Bake for 10–12 minutes, until cookies are set and spring back lightly when touched.
08 - Cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
09 - Beat butter until smooth. Gradually add powdered sugar and beat until fluffy. Add marshmallow crème, vanilla, and salt. Beat until smooth.
10 - Pair cookies by size. Pipe or spread 1–2 tablespoons of filling onto the flat side of one cookie, then sandwich with its pair. Serve immediately or chill 15 minutes to set.