01 - Preheat the 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.
03 - In a large bowl, beat butter and sugar together with an electric mixer until light and fluffy, about 2–3 minutes.
04 - Add egg and vanilla extract; beat until well combined.
05 - Mix in the red food coloring until evenly distributed.
06 - On low speed, alternately add the flour mixture and buttermilk in three additions, starting and ending with the flour. Mix until just combined; do not overmix.
07 - Using a tablespoon or small cookie scoop, drop 24 evenly-sized mounds of batter onto the prepared sheets, spacing them 2 inches apart.
08 - Bake for 10–12 minutes, or until cookies are set and spring back lightly when touched.
09 - Cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
10 - While cookies cool, prepare the filling: In a large bowl, beat the 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 - To assemble, pair cookies by size. Pipe or spread about 1–2 tablespoons of filling onto the flat side of one cookie, then sandwich with its pair.
13 - Serve immediately, or chill for 15 minutes to set the filling.