Preheat oven to 400°F. Melt butter over medium-high heat in a dutch oven or large, deep oven-safe skillet, such as stainless steel or cast-iron.
When butter is melted and the pan is hot, add kielbasa, onion and garlic, sauteing until softened, approximately 5-7 minutes.
Add in corn and rotel mixing well to incorporate with other ingredients and cook for an additional 1-2 minutes. Sprinkle flour over the entire mixture and stir to combine. Allow roux to cook for 1-2 minutes or until golden. Reduce heat to medium.
Whisk in chicken broth and salt and pepper, to taste, incorporating with the flour. Allow sauce to thicken, approximately 3 minutes. Add barbecue sauce and stir frequently until thick. Reduce heat to low and allow to simmer for 15 minutes. Remove from heat.
While kielbasa mixture is simmering, make the cornbread mixture by stirring the boxed cornbread mix, egg, milk and half of the cheese together in a medium sized mixing bowl. Mix until smooth, making sure there are no large clumps in the mixture.
Pour the cornbread mix over the top of the kielbasa mixture, spreading out so that it is evenly distributed over the top. Bake uncovered for 15 minutes. Remove from the oven and top with the remaining cheese. Bake an additional 10 minutes uncovered or until the cheese is melted and cornbread is fully cooked.
Allow the casserole to cool for at least 10 minutes before serving. Top with any optional toppings of your choice.