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 soda, and salt until well blended.
03 - In a large bowl, beat butter and sugar together until light and fluffy, about 2-3 minutes. Add the egg and vanilla, mixing until fully incorporated.
04 - Mix in the buttermilk and red food coloring until the batter is evenly colored and smooth.
05 - Gradually add the dry ingredients to the wet mixture, stirring just until combined. Be careful not to overmix.
06 - Using a tablespoon or small cookie scoop, drop rounded mounds of batter onto the prepared baking sheets, spacing 2 inches apart to allow for spreading.
07 - Bake for 10-12 minutes, or until cookies are set and spring back when lightly touched. Cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
08 - Beat the butter until smooth and creamy. Add powdered sugar and beat until fluffy. Mix in marshmallow creme, vanilla, and salt until smooth and spreadable.
09 - Once cookies are completely cool, spread or pipe the marshmallow filling onto the flat side of half the cookies. Top with the remaining cookies to create sandwiches.
10 - Store in an airtight container at room temperature for up to 2 days or refrigerate for extended freshness.