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