In A Saucepan, Saute The Onion, Celery And Garlic In Butter Until Tender. Stir In The Salsa, Ketchup, Brown Sugar, Vinegar, Worcestershire Sauce, Chili Powder, Salt And Pepper. Add Chicken; Stir To Coat. Bring To A Boil. Reduce Heat; Cover And Simmer For 15 Minutes. Serve About 1/3 Cup Chicken Mixture On Each Bun.