Preheat the oven to 350℉. Spray a 9X9-inch baking pan with baking spray. Line the bottom and 2 sides of the pan and spray again.
In a medium bowl, sift the flour, cocoa powder, cornstarch, and salt.
1 cup All-purpose flour, 1 cup Unsweetened cocoa powder, 1 TBSP Cornstarch, 1 tsp Salt
Using a mixer in a large bowl, beat the sugar, powdered sugar, eggs, and egg yolk on high speed for 2 minutes. It will be pale and fluffy. This is where the brownies get their rise.
1 3/4 cup White granulated sugar, 1/4 cup Powdered sugar, 3 Large eggs, 1 Egg yolk
Add the melted butter, oil, and vanilla and mix on low speed until combined. Add the dry ingredients and mix on low until combined. Take off the mixer and use a rubber baking spatula to mix the batter and make sure it is all mixed together. Add the chocolate chips and mix.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract, 1 cup Semi-sweet chocolate chips
Spread the brownie batter over the pan. Bake for 40-50 minutes. Bake until the edges are set and the middle is a little underdone. If you stick a toothpick in, it should be moist brownie come out, if it is raw batter, bake for another 5 minutes and check again.