Line an 8-inch square baking pan with parchment paper for easy removal (optional).
In a double boiler or microwave-safe bowl, melt the semi-sweet chocolate chips over low heat. Stir until completely smooth.
Once melted, gradually mix in half of the sweetened condensed milk, stirring continuously until fully combined.
Remove the chocolate mixture from the heat and add the peppermint extract and a pinch of salt. Fold in half of the crushed peppermint candies.
Pour this mixture into the prepared baking pan and spread it evenly.
In a separate bowl, melt the white chocolate chips in the same manner as the semi-sweet chocolate. Once melted, stir in the remaining sweetened condensed milk until smooth.
Pour the white chocolate mixture over the semi-sweet chocolate layer in the baking pan and spread evenly.
Sprinkle the remaining crushed peppermint candies on top for garnish.
Refrigerate the fudge for at least 2 hours, or until it is completely set.
Once set, cut the fudge into 12 squares and serve.