Bring a small pan with 1 inch of water to a boil over medium-low heat. Place the white chocolate in a small bowl and place over the simmering water. When it is almost halfway melted, remove the bowl from the heat to a towel. Continue to stir with a rubber spatula until completely melted. Set aside. Repeat with the bittersweet chocolate.
Pour the bittersweet chocolate onto a parchment-lined rimmed baking sheet and spread to 1/4-inch thick. Use a rubber spatula to drizzle the white chocolate over the bittersweet in zigzags and decorative patterns. Run the tip of a skewer through the chocolate in different directions to make swirls. Sprinkle with the toppings as desired. Refrigerate for about 30 minutes, then break into pieces.