Prepare the Pan
Line an 8-inch square pan with parchment paper, ensuring the paper extends over the sides to create a sling. This prevents sticking and makes the fudge easy to remove for cutting.

Melt the Dark Chocolate Base
In a small saucepan (or microwave-safe bowl), combine the semisweet chocolate chips, 2 tablespoons of the salted butter, and two-thirds of the can of sweetened condensed milk. Melt this mixture over low heat on the stovetop, stirring constantly, or in 5-second increments in the microwave. Remove from heat just before fully smooth and continue stirring until it is completely melted and glossy. Set aside.

Prepare the Peppermint White Chocolate
Repeat the melting process in a separate bowl or pan. Combine the 1 1/4 cups of white chocolate, the remaining 2 tablespoons of butter, and the remaining sweetened condensed milk. Melt gently, stirring constantly. Once melted, stir in the 1/4 teaspoon of peppermint extract and 2 tablespoons of the crushed candy canes. Mix well to combine.

Swirl and Set
Working quickly while both mixtures are still warm and liquid, use a spoon to dollop alternating portions of the dark chocolate and white chocolate mixtures into the lined pan until all of the fudge is used. Gently drag a toothpick or skewer through the dollops just once or twice to create a beautiful marble effect. Do not over-swirl.

Cut and Serve
Top the fudge immediately with the remaining reserved crushed candy canes. Refrigerate the pan until the fudge is completely set, which takes about 2 to 3 hours or overnight. Once firm, remove the fudge from the pan using the parchment paper sling and cut it into 36 bite-sized pieces. For perfect, clean slices, wipe your knife with hot water between every cut.