In a large mixing bowl combine the dry ingredients, flour, baking powder, salt, and cocoa powder. Whisk to combine, then set aside.
In another mixing bowl combine the wet ingredients, brown sugar, butter, buttermilk, vanilla extract, egg + egg yolk. Beat with an electric mixer until combined.
Pour the wet ingredients into the dry ingredients and beat with the electric mixer. Add in the red gel food coloring and beat again until combined.
On a parchment or silicone lined baking sheet, by the spoonfuls drop the batter in rows, be sure to leave plenty of room between each spoonful drop because the cookies will spread as they bake. Bake at 350°F for 10 minutes or until the tops of the cookies are no longer glossy. Let cool on a cooling rack.
While the cookies are cooling, make the marshmallow buttercream, follow the link below to the recipe for the marshmallow buttercream.
Using a piping bag with either a large star tip or a large circular tip, pipe the marshmallow buttercream on the bottom of half of the cookies. Then place the other half of the cookies over the buttercream to create a sandwich. Serve immediately, or store in an airtight container for up to a week.