In a medium bowl, whisk together flour, cornstarch, baking soda, baking powder, and salt until well combined. In a separate bowl, beat together softened butter, brown sugar, and granulated sugar until light and fluffy, about 3 minutes. Add peanut butter, vanilla extract, and an egg, mixing until smooth. Gradually incorporate the dry mixture, then fold in oats, chocolate chips, and M&M's. Chill this monster cookie dough mixture in the refrigerator for 40 minutes.

Preheat your oven to 325°F (160°C) and prepare a 9x9 inch baking pan by spraying it with non-stick spray and lining it with parchment paper. In a large bowl, sift together flour, cocoa powder, cornstarch, and salt. In another bowl, whisk together sugar, eggs, and an egg yolk for approximately 5 minutes until pale and fluffy. Slowly mix in melted butter, oil, and vanilla extract, then gently fold in the dry ingredients and chocolate chips.

Pour the brownie batter into the prepared baking pan, spreading it evenly. Bake the brownies for 15 minutes. From the chilled cookie dough, flatten small discs and place them atop the brownie layer. Return the pan to the oven and bake for an additional 25-30 minutes until the cookie layer is golden and the brownies are set but fudgy in the center.

Once baked, remove the pan and allow it to cool completely on a wire rack for about 30-45 minutes. This helps deepen the chocolate flavors and ensures the brownies hold their shape when sliced. After cooling, lift the brownies out of the pan using the parchment paper and slice into squares.