Preheat and Prep: Preheat your oven to 350°F (175°C). Line a 9-inch square pan with foil, allowing the foil to hang over the sides for easy removal. Spray the foil with cooking spray.

Make the Brownie Layer: Prepare the brownie batter as directed on the box. Spread the batter evenly in the prepared pan.

Bake the Brownies: Bake for 32 to 35 minutes, or until a toothpick inserted in the center comes out almost clean. The brownies should be set around the edges.

Cool Slightly: Cool the brownies in the pan for 10 minutes. This allows them to set up a bit before adding the cookie layer.

Make the Cookie Dough: While the brownies are cooling, prepare the sugar cookie dough as directed on the pouch for drop cookies. Stir in 1 cup of the green M&M’s®.

Layer the Cookie Dough: Drop half of the cookie dough mixture in chunks on top of the baked brownie layer. Add half of the chopped Mint OREO® Cookie pieces. Repeat with the remaining cookie dough and OREO® pieces.

Press and Top: Use the back of a spoon or an offset metal spatula to gently press the cookie dough together, forming an even layer. Top with the remaining ¼ cup of green M&M’s®, pressing them gently into the dough.

Bake Again: Bake for 23 to 28 minutes, or until the cookie layer is set. Cover the pan with foil for the last 10 minutes if necessary to prevent overbrowning.

Cool Completely: Cool the brookies completely in the pan (at least 2 ½ hours). This is crucial for allowing them to set up properly.

Cut and Serve: Once cooled, lift the brookies out of the pan using the foil overhang. Cut into 4 rows by 4 rows to create 16 bars.