Martha Washington’s Fudge Frosting

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 frosting 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
1/2×9-inch layers.