1.Line a 8-inch square pan with foil, making sure it goes up all four sides. Mist with non-stick spray. Preheat oven to 350 degrees.
2.In a medium saucepan, to make the brownie batter, melt the butter and chocolate over low heat, stirring until smooth. Remove from heat and whisk in 2/3 cup sugar, then the 2 eggs.
3.Mix in the flour, cocoa powder and salt, then 1 teaspoon vanilla and chocolate chips. Spread evenly in the prepared pan.
4.In a separate bowl, beat together cream cheese, egg yolk, 5 tablespoons sugar, and 1/8 teaspoon vanilla until smooth.
5.Distribute the cream cheese mixture in eight dollops on top of the brownie mixture, then take a dull knife or spatula and swirl the cream cheese mixture with the chocolate batter. Rap the pan on a countertop a few times to let the batter and swirl settle together.
6.Bake until the batter in the center of the pan feels just set, 40 minutes. Let cool, then lift the brownies out holding the foil and peel it away. Cut the brownies into squares.