Combine sugar, water, corn syrup, and salt in small saucepan. Bring quickly to a boil, stirring only until sugar is dissolved. Then boil, without stirring, until mixture forms a very soft ball in cold water (232 degrees F.). Cool to lukewarm ( 110 degrees F. ). Melt chocolate in medium saucepan over boiling water. Add butter and vanilla. Remove from boiling water. Add syrup gradually, stirring constantly. Continue stirring until smooth and thickened. Place again over boiling water and stir until frostin0g is softened and of right consistency to spread. Remove from boiling water and spread on cake. If necessary, place over hot water to keep soft while spreading. Makes enough frosting to cover tops and sides of two 7 1/2x9-inch layers.