Preheat the oven to 375 degrees F. Line baking sheets with parchment paper.
In a medium bowl, mix flour, cocoa powder, baking soda, and salt until combined.
In a separate medium bowl,beat butter and brown sugar together.
Then beat in the egg and vanilla.
Add buttermilk. Then gradually stir in the flour just until combined.
Divide the batter in half. Add red to one half, and blue to the other half until they reach your desired shade.
Spoon the cookie batter onto the baking sheet. Gently flatten each cookie with a greased hand. Bake for 7-9 minutes. Cool completely.
Mix butter, fluff, and vanilla together. Gradually stir in powdered sugar. Spread or pipe frosting onto half of the cookies.