Preheat oven according to brownie mix instructions and line an 8x8 baking pan with parchment paper.
Prepare and bake brownies in the pan following package directions, then allow them to cool completely.
Beat softened butter in a bowl until smooth; add powdered sugar and mix until combined with a dry texture.
Add mint extract, green food coloring, and 1 tablespoon heavy cream; beat until smooth and combined.
Adjust flavor and color as desired; add additional heavy cream and beat until the buttercream is fluffy.
Spread the mint buttercream evenly over cooled brownies and chill in the refrigerator or freezer to firm.
Heat 1 cup heavy cream until steaming but not boiling.
Pour hot cream over chocolate chips, cover, and let sit for 3 to 4 minutes, then whisk until smooth ganache forms.
Spread ganache over chilled buttercream layer and allow to set at room temperature for 45-60 minutes or refrigerate for about 15.
Lift brownies out of the pan using parchment flaps, cut into squares, and serve.