Preheat oven to 350°F. Line a 9x9 straight-sided metal baking pan with parchment paper, leaving overhang. Spray with baker's spray and set aside.

For the brownie layer, combine brownie mix, eggs, vegetable oil, and water in a large bowl. Whisk for 1-2 minutes until smooth.

Spread brownie batter evenly in the prepared pan. Bake for 30-33 minutes, or until a toothpick inserted comes out with moist crumbs. Do not overbake.

Remove brownies from oven, reduce temperature to 300°F. Let cool completely on the counter. While cooling, prepare the cookie dough layer.

Line a 9x13 quarter sheet pan with parchment paper. Spread flour evenly and bake for 5-7 minutes to heat-treat. Watch closely to prevent burning.

Remove heat-treated flour from oven and let cool completely.

In a large bowl, beat softened butter, brown sugar, granulated sugar, and vanilla extract with a mixer on medium speed for 1-2 minutes until light and fluffy.

Lower mixer speed to low, add heavy cream and salt. Mix until incorporated.

Add cooled, heat-treated flour to the cookie dough batter, ½ cup at a time, mixing until just combined and no streaks remain.

Stir in mini semi-sweet chocolate chips until evenly distributed.

Once the brownie layer is completely cool, use a large cookie scoop to place scoops of cookie dough evenly over the brownie surface.

Gently spread the cookie dough into an even layer with a small offset spatula, being careful not to press too hard and mix the layers.

Sprinkle mini M&M's evenly over the cookie dough and gently press them in so they lay flat.

Refrigerate for 30 minutes until cookie dough is chilled. Lift brownies out using parchment paper and slice to serve.