For the cream cheese topping: 8 ounces cream cheese 1 cup sour cream, at room temperature 2 tablespoons all-purpose flour 2 tablespoons granulated sugar 1 teaspoon pure vanilla extract ¼ teaspoon fine salt
Warm the cream cheese in the microwave or in a saucepan over low heat, whisking frequently. The cream cheese should be completely smooth with no lumps remaining. Transfer to a medium bowl and whisk in the sour cream, flour, sugar, vanilla, and salt. Set aside.
For the brownies: 3 sticks unsalted butter 1 cup plus 2 tablespoons all-purpose flour ¾ cup cocoa powder 2 ¼ teaspoons salt 1 ½ teaspoons baking powder 6 ounces unsweetened chocolate, chopped 1 ½ cup sugar 1 ½ cup brown sugar 5 large eggs 2 teaspoons vanilla extract
Preheat the oven to 350º F with a rack set in the center. Spray the pan. In a medium bowl, whisk the flour, cocoa, salt, and baking powder; set aside. Melt the butter and the unsweeted chocolate in the microwave. Transfer the chocolate-butter mixture to a large bowl. Whisk in both sugars and scrape down the sides with a rubber spatula. Add the eggs one at a time, whisking until incorporated and scraping down the sides as necessary. Stir in the vanilla. Sprinkle the flour mixture over the chocolate mixture and whisk until just combined. Remove ½ cup of the batter and set aside. Scrape the remaining batter into the prepared pan and smooth with an offset spatula. Dollop the cream cheese mixture evenly over the brownie batter. Smooth with a clean offset spatula. Dollop the remaining brownie batter on the cream cheese layer. Use a chopstick or the tip of a butter knife to swirl the batter and cream cheese. Do not overmix. Bake the brownies until a toothpick inserted in the center comes out clean, 50 to 55 minutes. Let cool completely before cutting.