1. Preheat oven to 325°F. Line an 8x8 pan with parchment paper.
2. Melt butter and chocolate chips together until smooth. Let cool slightly.
3. Whisk in sugar, eggs, and vanilla until glossy.
4. Fold in flour, cocoa powder, and salt. Set aside.
5. In a separate bowl, beat cream cheese, sugar, egg yolk, vanilla, and sour cream until smooth.
6. Spread 2/3 of brownie batter into pan. Add spoonfuls of cheesecake mix. Top with remaining brownie batter.
7. Use a knife to swirl gently into a marbled pattern.
8. Bake for 35–40 minutes or until edges are set and center slightly jiggles.
9. Cool at room temp, then chill for 3 hours before slicing.
10. Cut with a hot knife and serve with ice cream if desired.