preheat oven to 450°. Sprinkle chocolate over pizza crust leaving a 1 inch border. Bake for 6 to 8 minutes on baking sheet and lower one third of oven until chocolate is melted, spread evenly.
Cool slightly. Meanwhile, sift sugar and 1 tablespoon cocoa over yogurt. Mix well. Add whipped topping in three editions, mixing gently. Spoon over chocolate 1 inch from edge. Sift remaining cocoa powder over and garnish with shaved chocolate if desired.