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 until well combined.
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 to the butter mixture. Beat until well combined and smooth.
05 - Mix in the red food coloring until the batter is evenly colored with no streaks remaining.
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 in the center.
09 - Cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely before filling.
10 - While cookies cool, beat the softened butter in a large bowl until smooth and creamy. Gradually add powdered sugar and beat until fluffy.
11 - Add marshmallow crème, vanilla extract, and pinch of salt. Beat until well combined and smooth, about 2 minutes.
12 - Pair cookies by size. Pipe or spread about 1–2 tablespoons of filling onto the flat side of one cookie, then sandwich with its matching pair. Press gently to spread filling to edges.
13 - Serve immediately, or chill for 15 minutes to set the filling for easier handling.