Line an 8×8-inch baking dish with parchment, leaving overhang.
In a microwave-safe bowl, melt semi-sweet chocolate chips with butter in 30-second bursts. Stir until smooth. Spread evenly into the pan. Chill.
In another bowl, melt white chocolate chips with condensed milk in 30-second bursts, stirring until creamy. Stir in vanilla.
Divide mixture into two bowls. Tint one red and the other black.
Drop spoonfuls of each color over the base. Swirl with a skewer or knife.
Refrigerate at least 2 hours until firm. Lift, cut, and serve.
