Preheat the oven to 350°F (175°C). Grease the baking pan.
In a large bowl, whisk flour, cocoa powder, baking soda, and salt.
In another bowl, beat sugar, eggs, sour cream, oil, and vanilla until smooth.
Gradually mix in the dry ingredients. Stir in chocolate chips.
In a separate bowl, blend cream cheese, sugar, egg, and vanilla until creamy.
Pour half the batter into the pan. Spread the cream cheese mixture on top.
Add the remaining batter, spreading gently.
Bake for 50 minutes or until a toothpick inserted comes out clean.
Cool before slicing to maintain that gorgeous swirl.