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.
Note: The cornbread should be made a day or two in advance.