Preheat oven to 350°F. Combine cornmeal, flour, sugar, baking powder, and salt in a medium mixing bowl. In a separate bowl, combine milk, egg, and vegetable oil. Pour dry ingredients into wet, and mix until just combined, without overmixing. Pour into a 9x9" baking dish and bake for 25-30 minutes or until a toothpick comes out clean. Cool.
Once cooled, cut cornbread into 1" chunks. Arrange in a single layer on baking sheet and leave on counter to dry for up to 2 days. Alternatively, dry in oven set to 250°F for a couple hours.
For Cornbread Stuffing
Melt butter in a saucepan over medium heat. Add onion and celery. Cook for 7 minutes or until onions and celery are soft. Add poultry seasoning.
Place dried cornbread in a large bowl. Top with onion/celery mixture, scraping remaining butter out of pan.
Add chicken broth, a little at a time, until bread cubes are moistened, stirring between each addition. You will need more or less depending on how dried cornbread cubes are. Once cornbread cubes are moist but not soaked, pour mixture into a 9x9" baking dish and wrap tightly with foil.
Bake at 350°F for 45-60 minutes or until top is golden and celery and onions are cooked through. If desired, remove foil during last 10 minutes of cooking to brown the topping.