Old-Fashioned Hot Fudge Sauce

2 (1 ounce) squares unsweetened chocolate
1/2 cup corn syrup
Salt, to taste
1/2 teaspoon vanilla extract
1 teaspoon butter

Melt chocolate in top of double boiler over hot, but not boiling, water. Stir in corn syrup. Remove from heat. Add salt, vanilla extract and butter. Serve warm.