In the top of a double boiler, combine all ingredients accept the cornstarch. Heat over simmering water until the ingredients are warm to the touch (about 160F). Stir continuously during heating. As the sugar dissolves, it may form a crust.

Transfer the heated mixture to a glass bowl and cool to room temperature. Break apart the crusty mix and rub the mixture between your fingers so that it becomes granular again. Add the cornstarch and stir to mix. Use immediately or keep in a glass jar with a tight-fitting lid. Store in a cool dark place.