Line an 8x8 inch square baking pan with parchment paper, leaving overhang on the sides for easy removal.

In a microwave-safe bowl, combine semi-sweet chocolate chips, 1 tablespoon butter, and half the sweetened condensed milk. Heat in 30-second intervals, stirring until smooth. Stir in vanilla extract.

In a second bowl, combine white chocolate chips, remaining 1 tablespoon butter, and remaining condensed milk. Heat in 30-second intervals, stirring until smooth. Stir in peppermint extract.

Drop alternating spoonfuls of the dark and white chocolate mixtures into the prepared pan.

Use a knife or skewer to gently swirl the batters together to create a marble effect. Do not over-mix.

Immediately sprinkle crushed candy canes over the top and press down gently.

Refrigerate for 2-4 hours until firm. Remove from pan using parchment handles and cut into squares.