1. Line an 8×8-inch pan with parchment paper, leaving extra on the sides for easy lifting.

2. In a saucepan over low heat, melt the semi-sweet chocolate chips with 2/3 of the sweetened condensed milk, 2 tablespoons butter, vanilla extract, and salt. Stir until smooth.

3. In a separate bowl or saucepan, melt the white chocolate chips with the remaining sweetened condensed milk and 2 tablespoons butter. Stir in the peppermint extract until creamy.

4. Spread most of the chocolate mixture evenly into the prepared pan.

5. Drop spoonfuls of the white chocolate peppermint mixture over the chocolate layer, then add a few spoonfuls of the remaining chocolate mixture on top.

6. Use a butter knife or toothpick to gently swirl the two mixtures together for a marbled look.

7. Sprinkle the crushed peppermint candies over the top and lightly press them into the surface.

8. Refrigerate for 2 to 3 hours, or until fully set.

9. Lift the fudge from the pan, cut into squares, and serve.