Melt butter in a large stockpot over medium heat. Add onion, celery, mushrooms, black pepper, salt and garlic powder. Sweat the vegetables about 4 minutes. Add sherry, and continue cooking 1 minute, stirring frequently.
Gradually add flour to pan, stirring constantly about 3 minutes. Mixture will be thick. Make sure flour is thoroughly mixed in. Gradually add broth, whisking constantly, and bring to a boil. Reduce heat, add basil stem, and simmer 5 minutes, stirring frequently.
Remove from heat, and stir in 1 cup of shredded Parmesan and cream cheese. Whisk until cream cheese melts. Remove basil. Add pasta and chicken, stirring until blended.
Pour into prepared baking dish. Combine seasoned Panko with the remainder of the Parmesan and sprinkle evenly over the dish.
Bake at 350, or until lightly browned and bubbly. Remove casserole from the oven and let stand 15 minutes before serving.