Preheat your oven to 350°F (177°C). Line a 9×9‑inch pan with parchment paper or grease well.

In a saucepan or heat-safe bowl, melt the butter over low heat. Remove from heat and whisk in sugar until smooth. Add eggs one at a time, then whisk in vanilla, vinegar, and red food coloring until uniform.

In a separate bowl, combine flour, cocoa powder, and salt. Gently fold into the wet mixture until just combined—you want a fudgy consistency, not a cake.

In another bowl, beat the cream cheese and sugar until smooth, then mix in the egg and vanilla until creamy.

Pour most of the red velvet brownie batter into the prepared pan, reserving a few tablespoons. Drop spoonfuls of the cheesecake mixture evenly over the brownie layer. Dollop the reserved red batter on top. Use a knife or skewer to swirl, creating a marbled pattern.

Bake for 25–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet).

Let cool in the pan for about 20 minutes, then lift out and transfer to a wire rack. After reaching room temperature, refrigerate for 1–2 hours to set.

Slice into squares and serve. Store leftovers in an airtight container in the fridge for up to 5 days (or freeze for up to 2 months).