Line an 8x8 inch baking dish with parchment paper, leaving an overhang on the sides for easy removal later.
In a medium saucepan, combine the semi-sweet chocolate chips and sweetened condensed milk. Heat over low to medium heat, stirring constantly until the mixture is smooth and fully melted.
Remove the saucepan from heat and add the peppermint extract and a pinch of sea salt. Stir to combine.
Fold in half of the crushed peppermint candies into the chocolate mixture, saving the rest for topping.
Pour the chocolate layer into the prepared baking dish and spread it evenly.
In a separate small bowl, melt the white chocolate chips in the microwave in 30-second intervals, stirring in between until smooth.
Drizzle the melted white chocolate over the chocolate layer and use a toothpick or knife to swirl it gently for a marbled effect.
Sprinkle the remaining crushed peppermint candies on top for added flavor and decoration.
Refrigerate fudge for about 10 minutes or until fully set.
Once set, lift the fudge out of the dish using the parchment overhang. Cut into squares and serve.