Preheat your oven to 350 degrees F. Line a 9×13 inch baking pan with parchment paper, leaving an overhang for easy removal.
In a large bowl, mix the melted butter and 2 cups of sugar until combined.
Beat in the 4 eggs one at a time, then stir in 1 teaspoon of vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Stir in the chocolate chips. Spread half of the brownie batter into the prepared pan.
In a medium bowl, beat the softened cream cheese with 1/4 cup sugar, 1 egg, and 1/2 teaspoon vanilla until smooth.
Drop spoonfuls of the cream cheese mixture over the batter in the pan.
Spread the remaining brownie batter over the cream cheese layer.
Use a knife or skewer to gently swirl the layers together to create a marbled effect.
Bake for 30 to 35 minutes, or until a toothpick inserted near the center comes out with moist crumbs attached, not wet batter.
Let the brownies cool completely in the pan before lifting out using the parchment overhang and cutting into squares.
Notes
For the best swirl, ensure your cream cheese mixture is fully smooth before layering.
Do not overbake; slightly underbaking results in a fudgier texture.
Chill the brownies for at least one hour before slicing for clean cuts.