Prepare the Pan: Line an 8×8-inch baking pan with parchment paper, letting the edges hang over for easy removal.

Melt the Base Ingredients: In a medium saucepan, combine the white chocolate chips, sweetened condensed milk, and butter. Cook over low heat, stirring constantly, until the mixture is smooth and melted.

Create the Red Velvet Mixture: Divide the mixture into two parts. In one half, add red food coloring, vanilla extract, apple cider vinegar, and cocoa powder. Stir until well combined.

Create the Swirl: Pour the plain white fudge mixture into the prepared pan. Drop spoonfuls of the red velvet mixture over the white mixture. Use a knife or skewer to swirl the two mixtures together, creating a marbled effect.

Add Marshmallow Cream: Drop small dollops of marshmallow cream over the swirled fudge. Gently swirl again to incorporate the marshmallow cream into the fudge.

Chill: Refrigerate the fudge for at least 2 hours, or until fully set.

Slice and Serve: Once set, remove the fudge from the pan using the parchment overhang. Cut into squares and serve.