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.
Once the icing is in the piping bags, it’ll keep for several days. Use immediately or store in the refrigerator.