Heat granulated sugar and water in a saucepan until it reaches 245 degrees.

While the syrup is heating, mix egg whites on low until foamy with a whisk attachment.

Add cream of tarter then mix on medium to high until stiff.

Pour hot syrup in a slow stream into egg whites while the mixer is on high.

Beat for 8 minutes or until the mixture is cool.

To help the mixer's motor stay cool and prevent overheating place an ice pack on top of the motor and wrap ice packs around the sides of the bowl. You can tie them on with a thin dish towel.

Keep butter in the fridge until this point.

Remove the whisk attachment and put the paddle attachment on the mixer.

Add vanilla and butter in small cubes a bit at a time.

Beat until all butter is combined.

If it looks like it's separating or curdled or soupy looking, keep beating and keep the bowl cold if needed. The frosting will come together as long as you don't add the butter while the syrup/egg mixture is warmer than room temperature.

The frosting will be smooth and creamy.