Preheat oven to 325°F (163°C). If using boxed mix, bake brownies directly in a 9-inch springform pan and cool slightly.
Beat cream cheese and sugar until smooth. Scrape sides so there are no lumps.
Add eggs one at a time, mixing gently. Stir in sour cream, vanilla, flour, and salt until just combined.
Pour filling over cooled brownie crust. Smooth top and sprinkle chocolate chips or nuts if using.
Bake 40–50 minutes until edges set and center slightly jiggles. Cool 30 minutes, then chill at least 4 hours (overnight is best).