Preheat oven to 400 degrees. In a medium bowl, stir together all ingredients for cornbread. Pour into a lightly greased 9-inch cast iron pan or a 9-inch baking pan. Bake for 20 to 25 minutes. Before using, crumble into small pieces. Heat butter over medium heat in a large pan. Add celery and onion and cook until soft. Add sage, poultry seasoning, salt, and pepper to onion mixture. In a large bowl combine crumbled cornbread and toast. Whisk together milk and eggs and add to bowl. Stir in 2 cups of chicken broth. Stir in onion mixture. Mixture should be very moist. Add more broth if necessary. Transfer to a greased baking dish. Cut butter into small slivers and scatter on top of dressing. Bake at 350 degrees for 30 minutes, or until it turns light brown on top.