Ok, ok... its not icing sugar and it not exactly Fat, sugar or carb free, but pretty fukin close. I discovered this by accident.
I have only done this with Optimum Nutrition's Chocolate Casein Protein powder although I am sure it will work with most Casein powders.
You can do this in a shaker cup or a mixing bowl with a mixer.
Take one part Casein powder and add one even part water. Mix or shake, if shaking it in a cup you will hear it stop shaking almost instantly as it thickens right up.
If it does not thicken up and can still hear/feel it moving around in the cup, you need more casein powder. If it is too dry and powder still exists add a little more water.
Now if done correctly you will have what looks exactly like icing for a cup cake. I tastes good, but not great.... now all you do is add splenda to your sweetness liking and your set to go, tastes amazing!
Healthy protein packed ICING!