01 - Preheat 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 combined.
03 - In a large bowl, beat butter and sugar together until light and fluffy, approximately 3-4 minutes. Add egg and vanilla, mixing until fully incorporated.
04 - Mix in buttermilk and red food coloring until evenly blended and the batter reaches a uniform red color.
05 - Gradually add dry ingredients to the wet mixture, stirring just until combined. Do not overmix.
06 - Using a tablespoon or small cookie scoop, drop rounded mounds of batter onto 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 baking sheets for 5 minutes, then transfer to wire rack to cool completely.
08 - Beat butter until smooth and creamy. Add powdered sugar and beat until fluffy. Mix in marshmallow creme, vanilla, and salt until smooth and creamy.
09 - Once cookies are completely cool, spread or pipe marshmallow filling onto the flat side of half the cookies. Top with remaining cookies to create sandwiches, pressing gently.
10 - Store in an airtight container at room temperature for up to 2 days or refrigerate for extended freshness.