Preheat oven to 425°. In a 12-inch cast-iron skillet, melt ½ cup butter over medium-high heat. Pour butter into a medium bowl; whisk in buttermilk and 3 eggs. Place skillet in oven to preheat.
In a large bowl, whisk together cornmeal, flour, sugar, 2 teaspoons salt, baking powder, and baking soda. Pour buttermilk mixture into cornmeal mixture; stir until combined. Pour mixture back into skillet.
Bake until a wooden pick inserted in center comes out clean, 20 to 25 minutes. Invert cornbread onto a wire rack. Let cool; crumble.
Reduce oven to 375°. Wipe skillet clean. Melt remaining ½ cup butter over medium-high heat. Add celery, onion, and garlic; cook, stirring occasionally, until softened, about 5 minutes. Remove from heat.
In a large bowl, whisk together broth and remaining 5 eggs. Gently stir in crumbled cornbread, celery mixture, pecans, poultry seasoning, thyme, pepper, and remaining 2 teaspoons salt. Spoon cornbread mixture back into skillet.
Bake until top is golden brown and dressing is firm to the touch, 45 to 50 minutes.
Recipe by Southern Cast Iron at https://southerncastiron.com/cornbread-dressing/