Line and 8x8 or 9x9 pan with parchment paper.
Make brownies according to box directions.
Allow to cool.
Cream the butter and peppermint extract.
Slowly add in powdered sugar beating until incorporated.
Spread mixture on top of b and refrigerate for at least 30 minutes.
Open frosting container and microwave for 30 seconds.
Stir and pour about 2/3 of frosting over brownies.
Smooth with a knife as needed.
Sprinkle with crushed candy canes.
Refrigerate for a few minutes to set the frosting.