Preheat oven to 350 degrees. Grease 13×9 inch baking pan (or 11 x 7 for thicker brownies).
Prepare brownies according to box (or make your own). Pour brownies into prepared pan and bake for 19-23 minutes.
While brownies are baking, prepare peanut butter frosting. Cream the butter and peanut butter until smooth. Slowly blend in powdered sugar.
When brownies are baked and completely cooled (about 1 hour), frost with the peanut butter frosting. Sprinkle frosting evenly with entire bag of Chex Mix Muddy Buddies mixture.
Drizzle melted chocolate over the top of the brownies. Cut into squares and dig in.