Prepare and bake your boxed brownie mix according to the package instructions for your pan size. Let them cool completely.
Beat the softened butter in a bowl until smooth. Slowly add the powdered sugar, heavy cream, and peppermint extract. Mix until light and fluffy.
Spread the peppermint frosting evenly over the cooled brownies. Sprinkle the crushed candy canes on top.
Chill the brownies in the refrigerator for about 15 minutes to let the frosting firm up, then cut into squares.