Preheat the oven to 350℉. Spray a mini muffin pan with a baking spray. Set aside.
In a medium bowl, mix the flour, cocoa powder, cornstarch, and salt.
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.
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 and mix on low speed until combined.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract
Add in the dry ingredients and mix on low speed until just combined. Take off the mixer, and use a rubber spatula to gently mix the batter. Add in the chocolate chips.
1 cup Semi-sweet chocolate chips
Use a 1 Tablespoon (a small cookie scoop is too big) to scoop the batter into the pan. Use your fingers to smooth the top down.
Bake for 12-14 minutes. Bake until a toothpick inserted comes out with moist crumbs.
Let the brownies sit for 10 minutes before transferring to a cooling rack.