Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper for easy removal later.

In a mixing bowl, combine the melted butter and sugar until smooth. Add the eggs, one at a time, mixing well after each addition.

Incorporate the red gel food coloring, vanilla extract, and white vinegar. Mix until well combined.

In another bowl, whisk together the flour, cocoa powder, and salt. Gradually fold this into the wet mixture until incorporated.

For the cream cheese swirl, beat the softened cream cheese, granulated sugar, egg yolk, and vanilla extract together until smooth.

Pour half of the brownie batter into your prepared pan, spreading evenly. Dollop spoonfuls of the cream cheese mixture over the brownie batter, followed by the remaining brownie batter.

Use a knife to swirl the cream cheese through the brownie batter for a marbled effect.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let cool in the pan before slicing into squares.