Preheat oven as indicated on brownie box. Line an 8x8 square baking pan with parchment paper and set aside.

Prepare and bake brownies in an 8x8 square baking dish according to the package instructions.

Cool brownies completely.

Prepare the buttercream layer by beating butter until smooth, adding powdered sugar and mixing until combined, then adding mint extract and food coloring to taste.

Use an offset spatula to spread the mint buttercream over the cooled brownies. Refrigerate to firm.

Heat heavy cream, pour over chocolate chips, let sit, and whisk until smooth.

Spread ganache over the chilled, frosted brownies.

Allow the ganache to set before cutting into squares.