Combine sugar, corn syrup and water in 1-quart casserole. Cook in microwave oven on full power for about 3-4 minutes, or until it spins a thread when dropped from a spoon. (The mixture starts bubbling slowly while cooking in the microwave.)
Beat egg whites until stiff peaks form. Continue beating, pouring hot syrup in thin stream and then vanilla into egg whites. Beat until stiff peaks form. Spread on cake.
