Prep:
Preheat oven to 325°F. Line a 9x13 pan with parchment paper, leaving overhang. Lightly spray with nonstick spray.
Brownie Batter:
Melt butter in a large microwave-safe bowl. Whisk in sugar. Add eggs one at a time, whisking well after each. Stir in vanilla.
In another bowl, whisk together cocoa powder, flour, and salt. Gradually mix dry ingredients into wet mixture until just combined. Fold in chocolate chips. Spread 2/3 of batter evenly in prepared pan.
Cheesecake Batter:
Beat cream cheese until smooth. Gradually add sugar and beat until fluffy. Add eggs one at a time, mixing just until incorporated. Mix in vanilla and flour until smooth.
Assemble Swirls:
Pour cheesecake mixture over brownie layer. Drop spoonfuls of remaining brownie batter on top. Use a knife to swirl in a figure-8 motion, about 10 times.
Bake:
Bake for 40–45 minutes, until edges are set and center jiggles slightly. Check with a toothpick in the brownie part (should come out with moist crumbs, not wet batter).
Cool & Chill:
Cool completely in pan for 2 hours. Then refrigerate for at least 4 hours or overnight before slicing. For clean cuts, wipe knife between each slice.