01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk melted butter, granulated sugar, and brown sugar until blended. Add eggs, vanilla, vinegar, and red food coloring. Whisk until smooth.
03 - Sift flour, cocoa powder, and salt into the wet mixture. Fold gently until just combined; avoid overmixing.
04 - Reserve 1/4 cup of red velvet batter. Spread remaining batter evenly into prepared pan.
05 - Beat cream cheese, sugar, egg yolk, and vanilla until smooth and creamy.
06 - Dollop cheesecake mixture over brownie batter. Drop spoonfuls of reserved red velvet batter on top.
07 - Run a skewer or knife through the batters to swirl together for marbled appearance.
08 - Bake for 33-36 minutes until center is just set and toothpick inserted near edge shows moist crumbs.
09 - Cool completely in pan, then lift out using parchment overhang and cut into squares.