Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides, and lightly grease.
Step 2: For the Brownie Base: In a large microwave-safe bowl, melt the 5 tablespoons unsalted butter. Add 1 1/3 cups granulated sugar and whisk vigorously for about 1 minute until well combined and slightly shiny.
Step 3: Whisk in the 2 large eggs and 1 egg yolk one at a time, mixing well after each addition until the mixture is thick and glossy. Stir in the 1/3 cup oil and 1 tsp pure vanilla extract.
Step 4: In a separate bowl, combine 1/2 cup all-purpose flour, 3/4 cup unsweetened cocoa powder, 1 tablespoon cornstarch, and 1/4 tsp salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in 1/2 cup semi-sweet chocolate chips and 1/2 cup milk chocolate chips.
Step 5: Pour the brownie batter into the prepared pan and spread evenly. Set aside.
Step 6: For the Monster Cookie Topper: In a medium bowl, whisk together 1/2 cup all-purpose flour, 1 tsp cornstarch, 1/2 tsp baking soda, 1/8 tsp baking powder, and 1/4 tsp salt.
Step 7: In a separate large bowl, cream together 1/4 cup room temperature unsalted butter, 1/3 cup brown sugar, and 1/8 cup white granulated sugar until light and fluffy. Beat in 1/4 cup creamy peanut butter and 1 tsp pure vanilla extract.
Step 8: Add the 1 large room temperature egg and mix until just combined. Gradually add the dry ingredients to the wet ingredients, mixing until combined. Fold in 1/2 cup quick oats, 1/2 cup mini chocolate chips, and 1/2 cup mini M&Ms.
Step 9: Dollop spoonfuls of the monster cookie dough over the brownie batter in the pan. Gently spread and press the cookie dough to create an even layer over the brownie base, covering it completely.
Step 10: Bake for 28-32 minutes, or until the edges are set and a toothpick inserted into the cookie layer comes out with moist crumbs (not wet batter). The center may still look slightly soft.
Step 11: Remove from the oven and let cool completely in the pan on a wire rack before lifting out and cutting into bars. This is crucial for fudgy brownies and clean cuts. Enjoy!