Preheat the oven to 350℉. Spray a 9X9 inch baking pan with baking nonstick spray. Line the bottom and two sides of the pan with parchment paper. Spray again.
In a medium bowl, mix the flour, cocoa powder, cornstarch, and salt. Set aside.
1 cup All-purpose flour, 1 cup Unsweetened cocoa powder, 1 TBSP Cornstarch, 1 tsp Salt
Using a mixer, beat sugar, powdered sugar, eggs, and egg yolks on high speed for 2 minutes.
1 1/2 cups White granulated sugar, 1/2 cup Powdered sugar, 3 Large eggs, 1 Egg yolk
Add in the melted butter, oil, and vanilla. Mix on low until combined. Add in the dry ingredients and mix on low speed until just combined.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract
Take off the mixer, add chocolate chips, M&M's, and mini M&M's. Use a baking rubber spatula to mix these in.
1/3 cup Semi-sweet chocolate chips, 3/4 cup M&M's, 1/2 cup Mini M&M's
Pour the batter into the pan. Spread evenly. Optional: Top with extra M&M's
Bake for 35-40 minutes. Bake until the edges are set, but the middle is underdone. It will finish baking as it cools.
Place the hot pan on a cooling rack and cool before removing and cutting into the squares.