01 -
Turn your oven to 350°F and get it heating up. Take a 9x9 inch pan and give it a good coating of baking spray. Line the bottom and two sides with parchment paper, then spray again to make sure nothing sticks.
02 -
In a bowl, whisk together your flour, cocoa powder, cornstarch, and salt until they're well combined. Set this aside for now.
03 -
Using your mixer, beat both sugars, eggs, and the extra yolk on high for about 2 minutes until they're really well combined and a bit fluffy.
04 -
Pour in your melted butter, oil, and vanilla. Mix these in on low speed. Now gradually add your dry ingredients, mixing just until everything comes together - don't overmix!
05 -
Take the bowl off the mixer and fold in your chocolate chips and both sizes of M&M's using a spatula. Be gentle but make sure they're evenly distributed.
06 -
Spread the batter evenly in your prepared pan. If you want, sprinkle some extra M&M's on top. Bake for 35-40 minutes - you want the edges set but the middle still a little underdone. It'll finish cooking as it cools.
07 -
Put the pan on a cooling rack and let it cool completely before cutting into squares. Patience is key here!