2 ounces unsweetened chocolate
2 tablespoons corn syrup
1 tablespoon butter
1 teaspoon vanilla extract
1/3 cup boiling water
1 cup granulated sugar

Combine and melt butter and chocolate inn the top of a double boiler over boiling water, stirring to blend well. Add boiling water. Blend thoroughly. Add sugar and corn syrup. Place over direct heat. Cover. Let the sauce boil readily, but not too furiously, for 3 minutes. DO NOT STIR! Continue boiling, uncovered 3 more minutes, without stirring the sauce. Add vanilla extract just before serving.

For conventional Hot Fudge Sauce which does NOT harden on ice cream: After covering and boiling for 3 minutes, uncover, reduce the heat and simmer 2 more minutes, all without stirring the sauce.