Prep
Preheat oven to 350°F (175°C).
Line a 9x9-inch baking pan with parchment paper.
Make the Red Velvet Batter
In a large bowl, whisk together melted butter and sugar.
Add eggs and mix until combined.
Stir in red food coloring and vanilla extract.
Mix in cocoa powder and vinegar.
Fold in flour and salt until just combined. Do not overmix.
Make the Cream Cheese Swirl
In a separate bowl, beat cream cheese and sugar until smooth.
Add egg and vanilla extract.
Mix until creamy and lump-free.
Assemble
Pour about ¾ of the red velvet batter into the prepared pan.
Spread cream cheese mixture evenly over the batter.
Drop spoonfuls of remaining red velvet batter on top.
Use a knife to gently swirl for a marbled effect.
Bake
Bake for 30–35 minutes, until edges are set and the center has moist crumbs when tested.
Do not overbake for a fudgy texture.
Cool
Allow brownies to cool completely before slicing.
For clean cuts, refrigerate for 1 hour and use a warm knife.