Prep the Pan: Line an 8×8-inch square pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly coat with nonstick spray.

Make the Chocolate Base: In a heatproof bowl, combine chocolate chips, butter, and sweetened condensed milk. Set over a pot of simmering water and stir until melted and smooth. Remove from heat and stir in peppermint extract and salt.

Make the Peppermint Swirl Layer: In a separate heatproof bowl, combine white chocolate chips, butter, and condensed milk. Melt gently over a double boiler until smooth. Stir in peppermint extract and red gel food coloring to create streaks.

Create the Swirl: Pour about three-quarters of the chocolate mixture into the prepared pan. Spoon dollops of the peppermint mixture on top, then add the remaining chocolate mixture between them. Use a knife or skewer to gently swirl the layers.

Add Topping: Sprinkle crushed peppermint candies over the surface and press gently so they stick.

Chill: Refrigerate for at least 3 hours or until fully set. Lift from the pan and cut into squares.