Combine sugar, water, corn syrup, and salt in a saucepan; stir until well blended. Boil slowly without stirring until mixture will spin a long thread when a little is dropped from a spoon (hold the spoon high above saucepan), or reaches 238 - 242 degrees F (114 - 117 degrees C).
In a large bowl, beat egg whites with a mixer until they are stiff, but still moist. Pour hot syrup slowly over egg whites while beating. Continue until mixture is very fluffy, and will hold its shape. Add vanilla, and beat until blended. If icing does not seem stiff enough, beat in 2 or 3 tablespoons confectioners sugar 1 tablespoon at a time until stiff enough to hold its shape. Spread on cake.