Preheat the oven to 325°F. Spray an 8X8-inch baking 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.
2/3 cup All-purpose flour, 2/3 cup Unsweetened cocoa power, 1/2 TBSP Cornstarch, 1/2 tsp Salt
Using a mixer with a whisk attachment. Beat the sugar and eggs on high speed for 5 minutes. It will become light and pale in color. While it is mixing, melt the butter.
1 cup White granulated sugar, 2 Large eggs
Add the melted butter, oil, and vanilla and mix on low until combined. Add the dry ingredients and mix on low until combined.
4 TBSP Unsalted butter, 1/4 cup Oil, 1 tsp Pure vanilla extract
Take off the mix and mix using a rubber spatula. Save 1/4 cup of brownie batter to the side. Spread the brownie batter in the pan.