In a medium bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt until well combined. In a separate large bowl, beat the softened butter, brown sugar, and white sugar until light and fluffy. Mix in the peanut butter, vanilla, and eggs until smooth. Gradually fold in the dry ingredients, then stir in quick oats, mini chocolate chips, and mini M&M's. Chill the dough in the refrigerator for 40 minutes.

Preheat your oven to 325°F (163°C). Grease a 9x9 inch pan and line it with parchment paper. In a medium bowl, sift together the flour, cocoa powder, cornstarch, and salt. In a large mixing bowl, beat together sugar and eggs until light and fluffy. Add melted butter, oil, and vanilla extract, mixing until well combined. Fold in the dry ingredients, then incorporate semi-sweet chocolate chips. Pour the batter into the prepared pan and spread evenly.

Bake the brownie batter for 15 minutes. Flatten the chilled monster cookie dough into discs. After 15 minutes, remove the brownies from the oven and distribute the cookie dough evenly on top. Return to the oven and bake for an additional 20-25 minutes, until golden and set.

Allow the brownies to cool completely in the pan for at least 1 hour before lifting out and slicing into squares.