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

In a medium saucepan over low heat, combine the sweetened condensed milk, chocolate chips, butter, and salt (if using).

Stir frequently until the chocolate chips are melted and the mixture is smooth and creamy.

Remove from heat and stir in the peppermint extract.

Pour half of the chocolate mixture into the prepared pan and spread evenly.

Sprinkle half of the crushed candy canes over the chocolate layer. Pour the remaining chocolate mixture over the candy cane layer.

Using a knife or toothpick, gently swirl the remaining crushed candy canes into the fudge. Don't over-mix.

Refrigerate for at least 2 hours, or until firm. Once firm, lift the fudge out of the pan using the parchment paper overhang. Cut into squares and garnish with reserved crushed candy canes.