Heat water in microwave in measuring cup until boiling. Add gelatin, stirring with fork until dissolved. Pour gelatin into large bowl. Add yogurt, and whisk together until smooth. Add container of sugar-free Cool Whip; stir until smooth. Pour into prepared shortbread piecrust or, to skip additional calories or carbs, serve in dessert cups. Refrigerate 2 hours or overnight before serving. Garnish with additional sugar-free Cool Whip when serving.