Prepare the Cookie Dough: In a large mixing bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder, and salt until well combined. In a separate bowl, cream the unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 3 minutes. Mix in the peanut butter and vanilla extract, then add the egg, blending until smooth. Gradually fold in the dry ingredients, followed by the quick oats, mini chocolate chips, and M&M's. Chill the dough for 40 minutes.

Make the Brownie Base: Preheat your oven to 325°F (163°C). Grease a 9x9-inch baking pan with non-stick spray and line the bottom with parchment paper for easy removal. In another bowl, whisk together the flour, cocoa powder, cornstarch, and salt. In a mixing bowl, combine the white sugar, eggs, and egg yolk, beating until fluffy. Gradually mix in the melted unsalted butter, oil, and vanilla extract. Fold the dry mixture gently until just combined, then stir in the semi-sweet and milk chocolate chips.

Combine and Bake: Pour the brownie batter into the prepared pan, spreading it evenly. Bake the brownie base in the preheated oven for 15 minutes, until the edges appear slightly set. Meanwhile, flatten handfuls of the chilled cookie dough into discs. After 15 minutes, remove the brownies and layer the cookie discs on top, covering the surface evenly. Return to the oven and bake for an additional 20-30 minutes, checking for doneness by ensuring there is no jiggle in the center and covering with foil if the top browns too quickly.

Cool and Slice: Once baked, remove the pan from the oven and allow the Monster Cookie Fudgy Brownies to cool completely in the pan on a wire rack. Once cooled, lift the brownies out using the parchment paper, and place them on a cutting board. Cut into squares to serve, ensuring each piece has a perfect layer of chewy cookie on top of the fudgy brownie for a delightful treat.