Cook the cream and butter together in a heavy saucepan over moderate heat, stirring constantly, until the butter is melted and the cream comes to a low boil. Add the sugars and vanilla, and stir for a few minutes, until the sugar is dissolved. Add the salt and sifted cocoa. Reduce the heat, and stir briskly with a whisk until the sauce is smooth. Pour into gift jars. When cooled, cover and store in the refrigerator. Tell the recipients of the gift to reheat gently in a heavy saucepan or microwave. Makes 3 cups