Preheat the oven to 350℉. Spray a 9X9 inch baking pan with baking nonstick spray. Line the bottoms and two of the sides of the pan with parchment paper. Spray again.
In a medium bowl, mix the flour, cocoa powder, cornstarch, and salt. Set aside. Melt the chocolate bar in the microwave for 20 second intervals until melted.
3.5 oz Semi-sweet chocolate bar, 1 cup All-purpose flour, 1 cup Unsweetened cocoa powder, 1 TBSP Cornstarch, 1 tsp Salt
Using a mixer, beat the sugar, powdered sugar, eggs, and egg yolk on high speed for 3 minutes. It will become pale and fluffy.
1 1/2 cups White granulated sugar, 1/2 cup Powdered sugar, 3 Large eggs, 1 Egg yolk
Add in the melted butter, oil, vanilla, and melted chocolate. Mix on medium until combined. Add in the dry ingredients and mix on low speed until just combined.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract
Take the bowl off the mixer and use a rubber spatula to scrape the sides and bottom of the bowl. Pour the brownie batter into the prepared pan.
Bake for 33-37 minutes. Bake until a toothpick inserted comes out with moist crumbs. You want to underbake these, so it finishes baking as it cools.
Cool completely in the pan before removing and decorating.