Preheat the oven to 300 degrees F (150 degrees C).

Break spaghettini into 1 inch pieces. Cook according to package directions; drain.

Meanwhile, cook mushrooms in butter until softened. Cut cooked chicken into large pieces and mix with mushrooms. Stir in sour cream and condensed soup. Fold in spaghettini. Transfer mixture to a buttered casserole dish and sprinkle with Parmesan cheese.

Bake in the preheated oven for 40 minutes.