1. Line a large baking sheet with parchment. Pour melted semisweet chocolate onto parchment and spread into a 9x12 inch rectangle about 1/4 inch thick; refrigerate until hardened, about 20 minutes.
2. Pour melted white chocolate onto the semisweet layer and quickly spread evenly. Immediately sprinkle surface with pepitas, mini M&M's and nonpareils. Chill and break into pieces.