1.Heat the oven to 350°F.
2.Spray an 8-inch square baking pan with cooking spray.
3.Beat the cream cheese, 1 tablespoon butter and 1/4 cup sugar in a medium bowl at medium speed until smooth.
4.Add 1 egg, 1/2 teaspoon vanilla and 2 tablespoons sour cream to the mixture and beat until creamy. Stir in 2 tablespoons of flour; set the mixture aside.
5.Beat 1/4 cup butter, 1/2 cup sugar, 2 eggs, 1/4 cup sour cream and 1/2 teaspoon vanilla in a large bowl at low speed until mixed. Beat at medium speed until creamy.
6.Beat in 2/3 cup flour, baking powder and salt at low speed.
7.Beat in the chocolate at low speed until mixed; and then beat at medium speed until smooth and creamy.
8.Spread half of the chocolate mixture evenly in the pan. Top with the cream cheese mixture. Drop the remaining chocolate mixture over the cream cheese mixture filling by spoonfuls.
9.Pull a table knife through batters to marble.
10.Bake the brownies for 25 to 28 minutes or until the center is set. Place the brownies on a cooling rack; cool completely (about 2 hours), and then refrigerate until the brownies set. Store any leftovers in the refrigerator. Serve with a drizzle of chocolate sauce, if desired.