Step 1

Create a double boiler - Simmer about 2-inches of water in a saucepan. In a heat-proof mixing bowl large enough to fit on top of the saucepan - without touching the water - add the egg white, water, corn syrup, granulated sugar, and cream of tartar. Set the bowl on top of the saucepan and attach a thermometer to the side of the bowl (recommended - see notes).

Step 2

Using one of the whisk attachments as your whisk, constantly whisk the mixture until foamy and white. We want the sugar to dissolve and the temperature of the mixture to reach 160F degrees. This may take up to 4 minutes. If you don't have a thermometer, you can test if the sugar has dissolved by carefully rubbing a little of the egg white between your thumb and index finger - if done, you will not feel sugar granules.

Step 3

Remove the bowl from the heat, and add the vanilla extract. Using a hand-held mixer with whisk attachments, beat on medium speed until stiff peaks form, and the frosting is pillowy and glossy. This may take about 2 and 1/2 minutes.

Step 4

The frosting is ready to be used immediately. If storing, place in an airtight container and refrigerate for up to 2 days. Longer than that, the frosting may start to loosen and separate. I do not recommend keeping the frosting at room temperature for longer than 4 hours.