Preheat oven at 450°F. Grease cast iron skillet or large pan (with Crisco shortening) spreading on bottom and sides (liberally). Pour cornbread mixture in greased skillet or pan. Bake about 30 minutes or until brown
Pour water in large pan. Add margarine, salt & chicken. Cook chicken breast until tender, about 1 hour. Cut in pieces and set aside. Add chicken broth to liquid.
In Large bowl-break cornbread in small pieces. Add hamburger bun pieces, onion, sage and black pepper, and celery. Pour liquid from chicken over cornbread mixture. Taste to see if more salt is needed. Add more water and chicken broth if cornbread mixture absorbs all of liquid. Mixture should have excess liquid before baking, as baking will make it thicker. Add chicken pieces. Stir (lightly) do not over stir.
Pour into 11x13 pan. Bake about 1 hour or until brown.