Preheat oven to 375 F. Line 2 baking sheets with parchment or silicone baking mats. Set aside.
In medium bowl whisk flour, cocoa powder, baking soda, and salt together; set aside.
In large mixing bowl beat butter on medium for 30 seconds.
Add in the sugars, and beat until well-combined.
Beat in egg and vanilla.
Alternate adding flour mixture and buttermilk, mixing after each addition just until combined. Stir in food coloring.
Scoop out 1.5 tablespoons batter, spacing cookies about 2 inches apart on prepared baking sheets.
Bake 7-12 minutes, or until cookies are set.
Let cool 5 minutes on baking sheet.
Remove to wire rack to finish cooling.
Top cooled cookie with vanilla buttercream or cream cheese frosting, and then place another cookie on top.
Cover and refrigerate any leftovers.
Notes
Be sure to measure the flour properly by either weighing the flour or by aerating the flour, spooning it into your measuring cup, and then leveling.
The butter is at cool room temperature if you can dent it when you press on it, but the butter holds its shape otherwise. You don't want to use warm butter in this recipe.
Regular or low-fat ok.
Or 2 tablespoons red food color.
Nutrition values are estimates. Nutrition information for 1 serving is for 2 cookies without frosting.