In a large mixing bowl, combine all-purpose flour, cornstarch, baking soda, baking powder, and salt. In another bowl, cream together the room temperature unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 3 minutes. Mix in the creamy peanut butter, vanilla, and egg until well incorporated. Gradually add the dry ingredients followed by the quick oats, mini chocolate chips, and M&M's. Chill the cookie dough in the refrigerator for 40 minutes to firm up.

Preheat your oven to 325°F (163°C) and prepare a 9x9-inch baking pan by spraying it lightly and lining it with parchment paper for easy removal. In a medium bowl, whisk together all-purpose flour, unsweetened cocoa powder, cornstarch, and salt until combined. In a separate mixing bowl, beat the sugars and large eggs until fluffy, around 4 minutes. Add melted unsalted butter, canola oil, and vanilla, mixing until all ingredients are incorporated. Gently fold in the dry ingredients along with the semi-sweet and milk chocolate chips.

Spread the fudgy brownie batter evenly into the prepared baking pan, smoothing the top with a spatula. Pre-bake the brownie layer for 15 minutes; it should be slightly set but not fully cooked. After pre-baking, take the chilled Monster cookie dough and flatten it into discs. Carefully layer the discs on top of the partially baked brownie layer, lightly pressing them down to ensure they bond during baking.

Return the pan to the preheated oven and bake for an additional 20-25 minutes. The cookie layer should appear golden, and both layers should be set without any jiggle in the center. If using a toothpick, it should come out with a few moist crumbs.

Once baked, remove the brownies from the oven and allow them to cool completely in the pan on a wire rack. When fully cooled, use a sharp knife to cut them into squares.