Preheat oven to 350F and spray a 7×11 pan with baking spray.
Combine butter and chocolate chips in a large, microwave-safe bowl.
Microwave for 30 seconds. Stir well and microwave again for another 15 seconds and then stir well again. Repeat in 15 second intervals until chocolate and butter are completely melted and well-combined.
Add cocoa powder and stir well.
Add sugars, stir until completely combined.
Add eggs and then egg yolk, one at a time, stirring well after each addition.
Add vanilla extract and salt and stir to combine.
Add flour and stir until completely combined.
Reserve 1/4 cup of the batter and spread the rest of the batter evenly into prepared pan. Prepare your cheesecake topping.
Combine cream cheese and sugar in a medium-sized bowl. Use an electric mixer to beat until creamy.
Stir in egg and vanilla extract. Stir until well-combined.
Pour cheesecake over prepared brownie batter. Take your reserved 1/4 cup brownie batter (if it has become stiff just stir it with a spoon) and drop/drizzle over cheesecake.
Use a knife to swirl brownie batter and cheesecake together.
Bake on 350F for 25-30 minutes or until center is set and a toothpick inserted in the center comes out mostly clean with a few fudgy crumbs.
Allow to cool COMPLETELY before cutting and serving.