Make the brownie batter according to package instructions. In a oven proof baking pan, spray some oil. Add the prepared brownie mix batter and add ½ of m&m's. Using your fingers push the m&m's inside the batter. Now add the remaining m&m's and let them be on surface. This helps in getting m&m flavour in between the brownie as well as on top.
Bake 26-28 mins. Let it cool and cut in pieces.
Recipe by Naive Cook Cooks at https://www.naivecookcooks.com/dark-chocolate-mm-brownies/