In a saucepan over medium heat saute the onion, celery, garlic, and butter until the celery and onions are tender about 7 mins.
Stir in the salsa, ketchup, brown sugar, vinegar, Worcestershire sauce, chili powder, and salt and pepper. Add the chicken to the pot and stir to combine. Bring to a boil. Reduce heat, cover and simmer for 12-15 minutes.
To serve, portion the chicken on the toasted buns and serve immediately.
To freeze: cool the meat mixture and place in freezer safe containers. Will keep in freezer for up to 3 months. When ready to use thaw in refrigerator overnight. Heat mixture in a saucepan and add ¼ of water or more to the saucepan if necessary. Note: hamburger buns are not included in the nutritional information.recipe source: adapted from Taste of Home