Line an 8×8-inch pan with parchment paper, leaving overhang

Melt white chocolate chips, condensed milk, and butter over low heat, stirring constantly until smooth

Remove from heat and stir in peppermint extract and salt

Pour half of the white chocolate mixture into the pan

Melt semisweet chocolate chips with butter until smooth

Spoon chocolate mixture over the white chocolate layer

Dollop remaining white chocolate on top and gently swirl with a knife or skewer

Sprinkle with crushed candy canes if using

Refrigerate until firm, then lift out and cut into squares