For the filling: Preheat the oven to 350 degrees F.
Place chicken breasts skin-side down in a 3-quart saucepot. Add chicken bouillon and 2 cups water. Place on high heat and bring to a boil. Cover pot and reduce heat to simmer. Simmer for 45 minutes.
Remove chicken from pot with tongs, and set aside. Add condensed cream of chicken soup, celery, onions, butter, garlic powder and black pepper, if using, to pot. Whisk to incorporate and return to a simmer. Simmer uncovered until vegetables are tender, 20 to 30 minutes. Remove from heat.
Dissolve the flour in 1 cup of water in a small bowl. Stir until smooth and no lumps remain. Whisk this flour mixture into broth in the pot. Bring this all to a rolling boil, then boil for only 15 seconds. Remove from heat and set aside.
For the pie: Cut shortening with a fork into 2 cups flour in a medium bowl until shortening is pea-sized. Add buttermilk and gently mix just until no dry flour is visible. Turn mixture onto a work surface that has been dusted with the remaining 1/2 cup flour. Knead dough until a firm ball is achieved, about 20 times. Roll out dough to 3/8-inch thickness.
Shred the chicken into a 9-by-13-inch baking dish. Add frozen mixed vegetables and entire contents of saucepot. Stir with a spatula to incorporate all ingredients. Cover pot pie filling with rolled out crust. (Cover only the filling, not the sides of the baking dish.) Drizzle 1/2 cup melted butter evenly over crust. Bake for 1 hour. Let rest for 10 minutes before serving.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Matthew's Cafeteria, Tucker, GA