Place 1/2 cup butter in a 13- x 9-inch pan, and heat in oven at 425° for 4 minutes.
Combine white cornmeal and sugar in a large bowl; whisk in 2 eggs and buttermilk. Pour hot butter into batter, stirring until blended. Pour batter into pan.
Bake at 425° for 30 minutes or until golden brown. Cool and crumble into a large bowl.
Sauté chopped andouille sausage in a large skillet over medium-high heat 3 to 4 minutes or until lightly browned.
Add sliced green onions, diced celery, diced onion, and diced green bell pepper to skillet; sauté 5 minutes or until vegetables are tender. Spoon sausage mixture into bowl with cornbread mixture.
Melt remaining 1/2 cup butter in skillet; add diced mushrooms, and sauté 5 minutes. If desired, reduce sauté time to 3 minutes, and add 1 cup dry sherry to skillet. Cook, stirring often, until liquid is reduced by half.
Add mushroom mixture, chopped pecans, chopped parsley, and Creole seasoning to cornbread mixture in bowl.
Whisk together remaining 2 eggs and chicken broth; add to cornbread mixture, and stir gently until moistened. Spoon into 1 lightly greased 13- x 9-inch baking dish and 1 lightly greased 8-inch-square baking dish.
Bake, uncovered, at 350° for 30 minutes or until golden brown.
To make ahead: Chill prepared, unbaked dressing in the refrigerator overnight. Let dressing stand until room temperature, then bake as directed.
Note: For testing purposes only, we used White Lily Buttermilk Cornmeal Mix and Tony's Creole Seasoning.