In a small microwaveable cup, combine the water and coffee powder. Microwave on high powder for about 20 seconds, or until hot. Stir to dissolve the powder.
In the jar of a blender, combine the milk, yogurt, sugar substitute, reserved coffee mixture, and ice. Blend on high speed or ice crush setting for 30-60 seconds, or until smooth. Pour into a large glass. Garnish with whipped topping and dust with cocoa, if desired. Serve immediately with a straw.