1. Melt butter in heavy medium saucepan over medium heat; add onion. Cook until tender. Stir in ketchup, brown sugar, cocoa, Worcestershire sauce, tomato paste, vinegar, mustard and hot pepper sauce.
2. Heat to boiling, stirring occasionally; remove from heat. Serve warm as topping for grilled hot dogs, burgers, chicken or steak. If sauce starts to become too thick, add water 1 tablespoon at a time. Refrigerate leftovers. About 2 cups sauce.