In a mixing bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder, and salt until evenly combined.

In a separate bowl, cream together the melted and slightly cooled unsalted butter with brown sugar and granulated sugar until light and fluffy, about 3 minutes.

Mix in the peanut butter, followed by the vanilla extract and egg, blending until fully incorporated.

Gradually fold in the dry ingredients, along with the quick oats, mini M&Ms, and chocolate chips. Chill the dough for 40 minutes to firm up.

Preheat your oven to 325°F (163°C) and prepare a 9x9-inch baking pan by spraying it with non-stick spray and lining it with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, cornstarch, and salt until well combined.

In another bowl, beat the granulated sugar, eggs, and egg yolk together until fluffy, about 4 minutes.

Gradually add in the melted butter, oil, and vanilla extract, mixing until smooth.

Carefully fold in the dry ingredient mixture along with the semi-sweet and milk chocolate chips.

Pour the fudgy brownie batter into the prepared pan, spreading it evenly. Bake the brownies for 15 minutes or until the edges appear set but the center remains slightly underbaked.

While the brownies bake, flatten out the chilled cookie dough and distribute it evenly over the baked brownie layer.

Return the pan to the oven and continue baking for an additional 20-30 minutes, or until the cookie layer is lightly golden and the brownies are fully set.

Once baked, remove the brownies from the oven and allow them to cool completely in the pan on a wire rack.

For faster cooling, place the baking tray in the freezer for about 25 minutes.

Once cooled, slice into squares and enjoy.