Preheat the oven to 350 degrees. Line a 9x9 baking pan with parchment paper then coat with cooking spray.
Prepare the brownies by melting the butter and chocolate together in a saucepan over low heat, constantly stirring, until melted.
Remove from the heat and whisk in the sugar, eggs, and vanilla extract until smooth and shiny.
Add the flour and stir with a rubber spatula until combined.
Pour the chocolate mixture into the prepared baking pan; set aside.
Prepare the cheesecake by beating the softened cream cheese, sugar, egg yolk, and vanilla extract together with a mixer until creamy and smooth with no lumps.
Drop spoonfuls of cheesecake batter into the brownie batter.
Using a knife, swirl the two batters together until you get a marbled look.
Sprinkle the top with chocolate chips.
Place into the oven and bake for 30-35 minutes.
Remove from the oven and allow to cool for 2 hours before slicing. (Refrigerate to speed up process if needed).
Serve with ice cold milk. Enjoy.