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 in a mixing bowl. Add the powdered sugar and beat until combined (mixture will be dry).

Add mint extract, green food coloring, and 1 tablespoon of heavy cream. Beat until smooth and combined. Taste the frosting and adjust tint or add additional extract if desired. Add an additional tablespoon of heavy cream and beat until fluffy and smooth.

Use an offset spatula to spread the mint buttercream in an even layer over the top of the cooled brownies. Place in the refrigerator or freezer to firm the buttercream.

Heat the heavy cream in a microwave-safe bowl until steaming (do not boil).

Pour hot cream over the chocolate chips. Let sit for 3-4 minutes with a plate over the bowl to allow the chocolate chips to soften in the hot cream.

Whisk the chocolate gently until a smooth ganache forms.

Spread ganache over the chilled, frosted brownies.

Allow the ganache to set (either on the countertop for 45-60 minutes or in the refrigerator for about 15 minutes).

Lift brownies from tray by the flaps of parchment paper. Cut into even squares and serve.