Preheat oven to 350ºF. Line baking sheets with parchment paper.
In a mixing bowl, beat together butter, brown sugar, baking powder, baking soda, salt, and vanilla extract.
Beat in egg until smooth. Stir in cocoa until combined. Alternately, add flour and milk until combined. Stir in black food coloring.
Transfer batter to a piping bag. Pipe 1 tablespoon sized mounds onto the baking sheets. With a damp finger, lightly pat the tops of the cakes to flatten any peaks.
Bake 8-10 minutes until the cakes are firm to touch. Remove from oven and cool for a few minutes and transfer to racks to cool completely.
To assemble, pipe buttercream filling between two cookies. Store in the refrigerator.