In a medium mixing bowl, sift the flour and cocoa powder. Add in cornstarch and salt.
⅔ cup All-purpose flour, ¾ cup Dutch process cocoa powder, 1 tablespoon Cornstarch, ½ teaspoon Salt
Using a mixer with a whisk attachment, beat the sugar, eggs, and egg yolk. Beat on high speed for 5 minutes. Melt the butter while it mixes, so it slightly cools.
1 ⅓ cup White granulated sugar, 2 Large eggs, 1 Egg yolk, 5 tablespoon Unsalted butter
Add in oil, melted butter, and vanilla. Mix on low speed until just combined. Add in the dry ingredients and mix on low speed until mostly combined.
⅓ cup Oil, 1 teaspoon Pure vanilla extract
Take off the mixer, and use a rubber spatula to fold the batter together until combined. Fold in the chocolate chips.
1 cup Semi-sweet chocolate chips
Spread ¾ths of the batter on the bottom of the 9X9 pan. Pour the Oreo cheesecake batter on top of the brownie batter. Use a ½ Tablespoon to drop dollops of brownie batter on top of the cheesecake batter. Use a knife to slightly swirl the batter around.
Bake for 45-55 minutes. Bake until the edges and golden, and the middle has a slight jiggle.
Turn off the oven, prop the oven door slightly, and let it sit for 15 minutes to slightly cool. Transfer to a cooling rack and cool completely.
Cover with foil and chill in the fridge for 6 hours or overnight. Take them out of the pan and cut into squares.