Mint Buttercream Frosting:
Beat the softened butter until light and fluffy.
Add the sugar, 2 tbsp of milk, and mint extract. Stir until combined, then beat until smooth.
Stir in the green gel paste or food coloring until it is the shade of green you want.
Spread over the cooled brownies.
Place in the fridge for 30 minutes so that the buttercream can set before adding the chocolate layer.