Preheat the oven to 350℉. Spray a 9X9-inch square pan with baking nonstick spray. Line the bottom and 2 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 with a whisk attachment, beat the sugar, powdered sugar, eggs, and egg yolk 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 until combined.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract
Add in the dry ingredients. Mix on low speed until just combined. Add in the chocolate chips and mix until just combined.
1 cup Semi-sweet chocolate chips
Pour the brownie batter into the pan. Spread until even. Bake for 40-45 minutes. Bake until a toothpick inserted comes out with moist crumbs.
Let the brownies cool in the pan completely before taking out of the pan.