This frosting takes a little extra time, but the results are worth it.
1 Combine sugar, water, salt, and vinegar in a saucepan. Cook and stir over medium heat just until clear. Then cook without stirring until syrup spins a long thread when dropped from tip of spoon, or to 240 degrees F (115 degrees C).
2 Beat egg whites until soft peaks form. Add syrup in a very thin stream, beating constantly. After all syrup is added, continue beating until frosting will hold its shape. Mix in vanilla.