01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt until well blended.
03 - Beat butter and sugar with an electric mixer on medium speed until light and fluffy, about 2 to 3 minutes.
04 - Add egg and vanilla extract. Beat until fully incorporated. Mix in red food coloring until color is evenly distributed.
05 - On low speed, alternately add flour mixture and buttermilk in three additions, beginning and ending with flour. Mix until just combined, taking care not to overmix.
06 - Using a tablespoon or small cookie scoop, drop 24 evenly sized mounds of batter onto prepared sheets, spacing 2 inches apart.
07 - Bake for 10 to 12 minutes, until cookies are set and spring back lightly when touched in the center.
08 - Let cookies rest on baking sheets for 5 minutes, then transfer to wire rack to cool completely.
09 - Beat butter until smooth. Gradually add powdered sugar, beating until fluffy. Add marshmallow crème, vanilla, and salt. Beat until smooth and well combined.
10 - Pair cookies by size. Pipe or spread 1 to 2 tablespoons filling onto flat side of one cookie, then top with its matching pair. Serve immediately or refrigerate 15 minutes to set filling.