In a medium-sized bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt until well combined. Using a mixer, beat the butter, brown sugar, and granulated sugar on medium speed for about 1 minute, until the mixture is light and fluffy. Add the peanut butter and continue mixing for another minute. Next, add the vanilla extract and egg, mixing until fully incorporated. Scrape down the sides of the bowl and mix again for 20 seconds to ensure everything is evenly combined. Gradually add the dry ingredients to the wet mixture and mix until just combined. Fold in the oats, mini M&M’s, and mini chocolate chips. Cover the bowl and chill the dough in the refrigerator for 40 minutes.

As the cookie dough chills, prepare the brownie batter. Preheat your oven to 325°F (165°C). Grease a 9x9-inch baking pan with non-stick spray, then line the bottom and two sides with parchment paper to make removing the brownies easier. In a medium bowl, whisk together the flour, cocoa powder, cornstarch, and salt; set aside. In a separate large bowl, use a whisk attachment on your mixer to beat together the granulated sugar, eggs, and egg yolk for 5 minutes until very light and fluffy. While mixing, melt the butter and allow it to cool slightly. Once the egg mixture is ready, add the melted butter, oil, and vanilla extract, mixing on low speed until combined. Remove the bowl from the mixer and gently fold in the dry ingredients with a rubber spatula, being careful not to deflate the batter. Stir in both the semi-sweet and milk chocolate chips.

Pour the brownie batter evenly into the prepared pan and smooth it into the corners with a spatula. Bake for 15 minutes, just until the brownie top is set but not fully baked, preparing for the cookie dough layer.

Remove the cookie dough from the fridge. Take small handfuls and flatten each into a disc shape. Arrange these cookie dough discs evenly on top of the partially baked brownies, covering the surface.

Return the pan to the oven and bake for an additional 20 minutes. Then, loosely tent the brownies with foil to prevent the cookie topping from over-browning. Continue baking for another 5 to 10 minutes, or until both the cookie dough and brownie are fully set with no jiggle when gently shaken.

Remove the brownies from the oven and transfer the pan to a cooling rack. Allow them to cool completely to room temperature before cutting to ensure clean slices. Store the cooled brownies at room temperature.