1. In a small heavy saucepan, combine 1 1/2 cups sugar, corn syrup, and water. Heat over medium heat, stirring occasionally, until the sugar has dissolved. Rub a bit between your fingers to make sure there is no graininess. Raise heat to bring to a boil. Do not stir any more. Boil, washing down sides of pan with a pastry brush dipped in cold water from time to time to prevent the sugar from crystallizing, until a candy thermometer registers 230 degrees, about 5 minutes. (Depending on the humidity, this can take anywhere from 4 to 10 minutes.)

2. Meanwhile, in the bowl of an electric mixer fitted with the whisk attachment, whisk the egg whites on medium speed until soft peaks form, about 2 1/2 minutes. Gradually add the remaining 1/4 cup sugar. Remove the syrup from the heat when the temperature reaches 230 degrees (it will keep rising as pan is removed from heat). With the mixer on medium-low speed, pour the syrup in a steady stream down the side of the bowl (to avoid splattering) containing the egg-white mixture.

3. Beat frosting on medium until cool, 5 to 10 minutes. The frosting should be thick and shiny. Use immediately.