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