Preheat the oven to 350℉. Spray a 9-inch springform pan with nonstick baking spray. Line the bottom with parchment paper circle. Spray again.
In a medium bowl, sift the flour, cocoa powder, cornstarch, and salt. Set aside.
3/4 cup All-purpose flour, 1 cup Unsweetened cocoa powder, 1 TBSP Cornstarch, 1 tsp Salt
Using a mixer in a large bowl, beat the eggs, egg yolk, sugar, and powdered sugar on high speed for 3 minutes. This lets the sugar crystals melt into the eggs.
3 Large eggs, 1 Egg yolk, 1 1/2 cup White granulated sugar, 1/2 cup Powdered sugar
Add in the melted butter, oil, and vanilla and mix on low speed until combined. Add in the dry ingredients and mix on low speed until just combined. Take off the mixer and use a baking rubber spatula to make sure it is all mixed together.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract
Pour the brownie batter into the pan. Bake for 25 minutes. It will appear just set on top.