In a medium bowl, combine the flour, cornstarch, baking soda, baking powder, and salt to create a dry mixture. In a larger bowl, cream together the unsalted butter, brown sugar, white sugar, and creamy peanut butter until fluffy and light. Add the vanilla extract and egg, mixing until well combined, then gradually incorporate the dry mixture. Stir in the oats, mini chocolate chips, and mini M&Ms, then chill the dough in the refrigerator for 40 minutes.

Preheat your oven to 325°F (163°C) and line a 9x9 inch baking pan with parchment paper, lightly greasing the sides. In a medium bowl, whisk together the flour, cocoa powder, cornstarch, and salt. In a mixer, beat together the sugars and eggs for about 5 minutes until fluffy and pale. Mix in the melted unsalted butter, oil, and vanilla, then fold in the dry ingredients. Pour the brownie batter into the prepared pan and spread it evenly before baking for 15 minutes to set slightly.

Once the brownie base has set, flatten the chilled monster cookie dough into discs and evenly layer it over the partially baked brownies. Gently press the cookie dough down to ensure it adheres to the brownie layer. Bake the assembled dessert for an additional 20-30 minutes, or until the top no longer jiggles and is a light golden brown. Allow the pan to cool completely before proceeding to slice into decadent squares.