Preheat oven to 350°F. Spray an 8x8 inch baking dish with cooking spray, or line with parchment paper.
Place the melted butter, 1 cup of sugar, vanilla, cocoa powder, salt, food coloring, vinegar, and 2 eggs in a large bowl. Whisk until combined. Stir in the flour and walnuts just until combined.
Reserve ⅓ cup of the brownie batter for the top. Pour the remaining batter into the prepared pan.
In another bowl, stir together the softened cream cheese, remaining ¼ cup sugar, and remaining egg. Carefully spread the mixture over the brownie batter in the pan.
Dollop the reserve ⅓ cup of batter over the top in small spoonfuls. Drag a skewer or knife through the batter to create a swirl pattern.
Bake for 25–30 minutes, or until a toothpick comes out clean. Allow to cool completely, slice, and serve.
Notes
To get the cleanest cuts, let the brownies cool completely and refrigerate for 2 hours before slicing. Store brownies in the refrigerator. You can also freeze them for up to 3 months.If you're using gel food coloring, start with 2–3 drops and add more as needed.