Heat gas or charcoal grill. Soak twine in water 10 minutes to prevent burning. Meanwhile, in 2-quart saucepan, melt butter over low heat. Stir in brown sugar, mustard and chili powder. Cook 2 to 3 minutes, stirring frequently, until mixture comes to a boil. Pour 1/4 cup sauce into small bowl; reserve for serving. Leave remaining sauce in saucepan for basting pork.