Preheat oven to 325°F. Grease a metal 9- x 13-inch baking pan and set aside.

In a medium saucepan, melt butter until bubbling but not brown. Turn off heat, add chocolate chips, and stir to coat. Let sit for 5 minutes. Stir again until the chocolate is completely melted and combined with the butter. Transfer to a large mixing bowl and add, brown sugar, 3/4 cup granulated sugar, 4 eggs, and 2 tsp. vanilla. Whisk until well combined. Add 1/2 tsp. salt, cocoa powder, and flour and stir together until well combined. Pour into prepared baking pan.

With a mixer, beat together cream cheese, 1/3 cup sugar, 1/2 tsp. vanilla, 1/8 tsp. salt, and 1 egg until well combined, about 3 minutes. Spread over top of brownie batter and using a knife, draw swirls across the surface of the cream cheese by running the tip of the knife about 1/2 inch deep into the brownie batter as you move it around. Bake until set, about 30-35 minutes. Allow to cool completely before cutting.