Grease an 8×8-inch baking pan and line it with parchment paper, leaving an overhang on two opposite sides for easy removal.
Place the white chocolate chips and sweetened condensed milk in a large microwave-safe bowl.
Microwave in 15–20 second intervals, stirring well after each interval, until the chocolate is completely melted and the mixture is smooth and fully combined (total time will vary by microwave).
Stir in the peppermint extract and the sea salt until evenly incorporated.
Pour the mixture into the prepared pan and spread it into an even layer. Sprinkle the crushed peppermint candies evenly over the top and press lightly so they adhere.
Allow the fudge to cool completely at room temperature, then chill in the refrigerator if needed until fully set.
Use the parchment overhang to lift the fudge from the pan and cut into 1-inch pieces.