In a one quart saucepan, melt butter and chocolate. In a separate saucepan, bring water to a boil and add to chocolate. Next, stir in sugar and corn syrup and simmer for 8 minutes. Keep sauce warm until ready to serve. Yields about one cup. Note: This recipe is used in the preparation of another recipe in this cookbook, "Dessert Tostada, " but can be prepared separately and used to top other desserts.