Blend sugar, cocoa & salt in large thick saucepan. Stir in milk & corn syrup. Place over moderate heat stirring until sugar is dissolved. Wipe candy crystals from side of pan as necessary; cook without stirring until frosting reaches 232 or very soft ball stage. Remove from heat. Add butter; without stirring, cool to 110 or lukewarm. Add extract. Stir continuously until creamy spreading consistency is reached.