Adjust the oven rack to the 2nd level position (just above center) and preheat the oven to 350ºF. Spray an 8 inch square baking pan with a flour-based baking spray. Line with parchment paper sling, if desired then lightly respray.
Using a hand mixer or stand mixer fitted with the whisk attachment, beat the cream cheese on medium speed until smooth and creamy. Add in the sour cream and granulated sugar, mixing until fully combined. Fold in the chocolate chips with a spatula. Set to the side.
In a small bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
Microwave the dark chocolate and unsalted butter together in 45 second to 1 minute increments at 50% power, stirring each time, until smooth and melted. Stir in the unsweetened cocoa powder. Set to the side.
In a large bowl, whisk together the eggs, vanilla extract and granulated sugar. Scrape down the sides of the bowl, if needed.
Using a whisk, add in the melted chocolate mixture. Then fold in the whisked dry mixture with a spatula stirring until just combined.
Reserve ½ cup of the brownie batter for topping.
To the prepared 8 inch square baking pan, add the brownie batter (minus the reserved ½ cup) and spread out with a spatula.
Top with the chocolate chip cream cheese filling.
Microwave the reserved brownie batter for 15-30 seconds then, with a spoon, add dollops of the warm batter over the top of the chocolate chip cream cheese filling. With a butter knife, swirl through the brownie batter and cream cheese filling to create a marbled pattern. Top with extra chocolate chips, if desired.
Bake for 30-35 minutes at 350ºF or until an inserted toothpick (the brownie parts only) comes out with only brownie crumbs.
Let the brownies cool in the pan for about an hour before cutting them.
Keep leftover brownies in a tightly sealed container in the refrigerator for up to 3 days. Brownies can be kept in the freezer by wrapping them individually in plastic wrap then storing them in a well sealed container for up to 1 month.