Preheat oven to 350 degrees. Line baking sheet with parchment paper. Whisk flour, baking soda, and salt in medium bowl. Whisk melted butter, both sugars, and vanilla in large bowl until smooth. Stir in flour mixture until just blended. Stir in 1 cup chopped white chocolate and 1/2 cup crushed peppermint candy. Transfer dough to prepared sheet. Press dough into 14x8-inch rectangle, about 3/8-inch thick. Bake cookie until top is firm and dark golden, about 30 minutes. Cool on sheet 10 minutes. Transfer to rack; cool completely. Stir remaining white chocolate in top of double boiler over barely simmering water until melted and smooth. Using small spoon, drizzle about half of melted chocolate in thin lines over cooled cookie. Sprinkle remaining crushed peppermint candies over chocolate. Drizzle remaining white chocolate over top. Let stand until white chocolate sets, about 1 hour. Break cookie into irregular 2- to 3-inch pieces. (Can be made 2 days ahead. Store in airtight container at room temperature.)

Makes about 24 pieces.