01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined.
03 - In a large bowl, beat the butter and sugar until light and fluffy, about 2 minutes. Add the egg and beat until fully incorporated.
04 - Mix in the buttermilk and vanilla extract. The batter may appear curdled, which is normal.
05 - Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
06 - Using a cookie scoop or tablespoon, drop rounds of batter (about 2 tablespoons each) onto the prepared baking sheets, leaving 2 inches of space between each.
07 - Bake for 10 to 12 minutes, or until the tops spring back when gently pressed. Cool completely on a wire rack.
08 - Beat the butter until creamy. Add the powdered sugar and beat until smooth. Mix in the marshmallow creme, vanilla, and salt until fully combined and fluffy.
09 - Spread or pipe a generous dollop of filling onto the flat side of half the cookies. Top each with another cookie, flat side down, to form sandwiches.
10 - Serve immediately or store in an airtight container for up to 3 days.