Preheat the oven to 350 degrees and prepare the brownie mix according to the package instructions.
Grease a 9x13 baking dish and spread half the batter across the bottom of the dish.
Sprinkle half the M&Ms and half the chopped candy over the top of the batter and top with remaining batter.
Spread the remaining candy over the top of the brownies.
Bake in the preheated oven according to the instructions on the box.
Allow to cool completely before slicing and serving.
Notes
Store any leftover brownies in an airtight container at room temperature to keep them moist and chewy. They'll last like this for several days — if they don't get eaten first!