• Beat the egg whites and cream of tartar in the bowl of a stand mixer with a paddle attachment until foamy.
• Add the powdered sugar and gradually increase the speed to high.
• Mix for 5 minutes, or until the icing is very thick.
• Use the icing as-is or tint it with gel food coloring. It will start to harden almost immediately, so be sure to work quickly.  
• Transfer to piping bags fitted with a couplers and various piping tips.
• Once the icing is in the piping bags, it’ll keep for several days.  Use immediately or store in the refrigerator.

Raw Egg White Alternative:

• As an alternative to raw egg whites, combine the meringue powder and warm water in the stand mixer and whisk until foamy.  Continue with step #2.