In the top of a double boiler or in a clean stainless steel bowl set over a pot of simmering water, combine the sugar, corn syrup, water, and egg whites. Make sure the simmering water doesn’t touch the bottom of the bowl or double boiler – you only need an inch or two of water in the pot. Beat the sugar mixture with an electric mixer on high speed for about 7 minutes, until it stands in billowy peaks. Remove it from the heat and beat in the vanilla.

To make Seafoam (Brown Sugar) Frosting: Substitute packed dark or golden brown sugar for the white sugar.