In a medium bowl, mix the flour, cocoa powder, cornstarch, and salt. Set aside.
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 2 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. Mix on low 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
Pour half of the brownie batter into the pan. Spread evenly. Use a large cookie scoop. Scoop all of the cheesecake batter on top of the brownie batter. Place the dollops around randomly. Scoop the brownie batter on top in randomly placed dollops.
Use a butter knife to very gently swirl it together. You want to be careful to not over swirl the batters.
Bake for 40-50 minutes. Bake until a toothpick inserted comes out with moist crumbs. If it raw batter, bake for another 5 minutes before testing again.
Cool the brownies (in the pan) on a cooling rack until completely cool. Place in the fridge and chill for at least 3 hours. Cut the brownies into 16 squares.