01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar until light and fluffy. Add egg and vanilla, mixing until combined.
04 - Mix in buttermilk and red food coloring until evenly blended.
05 - Gradually add dry ingredients to wet mixture, stirring just until combined.
06 - Drop rounded tablespoons of batter onto prepared baking sheets, spacing 2 inches apart.
07 - Bake for 10-12 minutes until set and springy when touched. Cool on sheets for 5 minutes, then transfer to wire rack.
08 - Beat butter until smooth. Add powdered sugar and beat until fluffy. Mix in marshmallow creme, vanilla, and salt until smooth.
09 - Spread filling onto flat side of half the cookies. Top with remaining cookies to form sandwiches.
10 - Store in airtight container at room temperature for up to 2 days or refrigerate for extended freshness.