Add the butter to the skillet with the bacon drippings. Stir in the diced onion (if using) and cook for 3–4 minutes, until softened. Add the diced jalapeños and cook another 2–3 minutes, just until slightly tender but still bright. Remove from heat and let cool slightly.
In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Mix until smooth. Stir in the garlic powder, onion powder, smoked paprika (if using), salt, and black pepper.
Fold the cooked chicken into the cream cheese mixture. Add about two-thirds of the cooked jalapeño-onion mixture and half of the cooked bacon, stirring until everything is evenly combined.
Stir in 1 cup of the shredded cheddar and 1/2 cup of the shredded mozzarella. Mix just until the cheese is evenly distributed throughout the chicken mixture.
Transfer the mixture to the prepared baking dish and spread it into an even layer with a spatula.
In a small bowl, combine the remaining 1/2 cup cheddar, remaining 1/2 cup mozzarella, and the grated Parmesan. Sprinkle this cheese mixture evenly over the top of the casserole.
Scatter the remaining cooked jalapeños and bacon over the top for extra flavor and texture.
Bake in the preheated oven for 20–25 minutes, or until the casserole is hot and bubbly and the cheese on top is melted and lightly golden around the edges.
Remove from the oven and let the casserole rest for about 5–10 minutes to set slightly. Garnish with chopped parsley or green onions, if desired, then slice into portions and serve warm.
