Preheat & Prepare: Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper and grease it lightly.
Make Brownie Batter:
Melt butter and let it cool slightly.
Mix butter and sugar until it resembles wet sand.
Add eggs one at a time, whisking after each addition, until mixture becomes glossy.
Stir in vanilla extract.
In a separate bowl, whisk together flour, cocoa powder, and salt.
Gradually fold dry ingredients into the wet mixture until just combined.
Stir in chocolate chips. Set aside.
Make Cheesecake Batter:
Beat softened cream cheese until smooth and lump-free.
Add sugar and beat until fluffy.
Mix in the egg and vanilla until just combined.
Stir in flour and mix briefly.
Assemble:
Spread most of the brownie batter into the prepared pan.
Spoon the cheesecake mixture on top in dollops.
Drop remaining brownie batter between the cheesecake dollops.
Use a knife to swirl gently in a figure-eight pattern.
Bake: Bake for 35-40 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.
Cool & Serve: Let brownies cool completely (at least 2 hours) before slicing. For cleaner cuts, chill in the fridge first.