Line large cookie sheet with waxed paper. Spread walnuts out in a single layer, close together, on the waxed paper; set aside.
Place dark chocolate in a 1 1/2-quart microwave-safe bowl. Microwave on medium (50%) 2 minutes; stir. Microwave on medium 30 seconds to 1 minute longer, or until chocolate can be stirred smooth and is glossy, stirring twice. Immediately pour over walnuts, spreading out to a thin, even layer. Let stand until firm.
Place white chocolate in a 1-quart microwave-safe bowl. Microwave on medium 1 1/2 to 2 1/2 minutes or until white chocolate can be stirred smooth, stirring twice. With the tip of a metal spoon or a pastry bag fitted with a small writing tip, drizzle white chocolate over dark in a zig-zag pattern. Let stand to set or refrigerate; break into pieces. Store in airtight container with waxed paper between layers in cool, dry place.Servings: about 1 1/2 pounds candy