Preheat oven to 325F. Spray a 9X9 pan with a non-stick spray.
In a small bowl, sift together flour, cocoa powder, salt, and cornstarch. Set aside
¾ cup All-purpose flour, 1 cup Unsweetened cocoa powder, ½ teaspoon Salt, 1 ½ tablespoon Cornstarch
Using a mixer, combine sugar, eggs, and egg yolk. Turn the mixer to high and beat for 5 minutes. It will become light and fluffy. While mixing, melt butter in the microwave and let it sit for a few minutes to cool down slightly.
2 cup White granulated sugar, 3 Large eggs, 1 Large egg yolk, ½ cup Unsalted butter
Add in butter, oil, and vanilla. Mix on low until combined. Take bowl off mixer. Add in the dry ingredients. Use a rubber spatula to very slowly fold in the dry ingredients. Pour in semi-sweet, milk, and mini chocolate chips and fold in.
2 teaspoon Pure vanilla extract, ½ cup Oil, ½ cup Semi-sweet chocolate chips, ½ cup Milk chocolate, ⅓ cup Mini chocolate chips
Pour batter into pan. Using a rubber spatula, make sure it is even through out pan. Bake for 45-50 minutes. The brownies are done when edges are done but the middle isn't set yet. Brownies should appear underdone. They cook more when sitting out. Let sit on counter until top is room temperature.
Right before serving, cut brownies into 16 squares. Sprinkle powdered sugar on top.
Powdered sugar for dusting