Stir in butter, then boiling water, then sugar and salt; cook stirring occasionally, for 15 minutes.
Remove from heat and stir in vanilla.
The sauce is best when served warm - Either serve immediately or reheat in double boiler for 10 minutes or microwave for 1 minute.
To store pour into sterilized glass jars.
Allow sauce to cool before refrigerating; it will keep for 6 months in the refrigerator.
Variations: Add any of the following flavorings in place of or in additon to the vanilla: : 1 teaspoon orange extract or 1/3 cup orange juice : 1 teaspoon mint extract : 1/8 cup crushed strawberries or rasberries, chopped raisins or nuts : 1 tablespoon Creme de Menth, Coffe liqueur, Spice orange Cordial or other : flavored liquer : 1 teaspoon instant coffee