In a mixing bowl, combine flour, cornstarch, baking soda, baking powder, and salt, whisking until well mixed. In another bowl, cream together softened butter, brown sugar, and granulated sugar for about 3-4 minutes until fluffy. Add peanut butter, vanilla extract, and egg, mixing until smooth. Gradually fold the dry mixture into the wet ingredients, then stir in oats, mini chocolate chips, and mini M&M's. Chill the dough for 40 minutes to firm it up.

While the cookie dough chills, preheat your oven to 325°F (163°C). Grease a 9x9 inch baking pan with non-stick spray and line it with parchment paper, allowing some overhang for easy removal.

In a separate bowl, whisk together flour, cocoa powder, cornstarch, and salt until evenly combined. In a stand mixer, beat together white granulated sugar, eggs, and egg yolk for approximately 5 minutes until thick and pale. Gradually mix in melted butter, oil, and vanilla extract, ensuring a smooth batter.

Pour the brownie batter into the prepared pan, spreading it evenly with a spatula. Bake in the preheated oven for 15 minutes.

Once the brownie base is done, remove it from the oven and let it cool slightly, about 5 minutes. Take the chilled cookie dough and flatten it into thin discs using your hands or a rolling pin. Carefully layer these discs over the brownie base.

Return the pan to the oven and bake for an additional 20-30 minutes. Check for doneness by inserting a toothpick into the center.

Once baked, remove the pan from the oven and let the Monster Cookie Fudgy Brownies cool completely at room temperature, about 1-2 hours.