Preheat oven to 375°F. Butter large casserole dish.
Melt butter in large saucepan. Add flour and whisk until it forms a thick paste. Add milk a little at a time while whisking continuously to form a sauce.
Add salt, pepper, mustard, paprika and cheddar cheese (reserving ½ cup of cheese for topping) and whisk until cheese is melted and sauce is smooth. Taste and adjust seasonings as needed.
Bring large pot of salted water to boil. Add pasta and cook until al dente. Drain and add pasta to the cheese sauce, stirring until well coated.
Dump pasta and sauce into prepared casserole dish.
For the topping, combine melted butter, bread crumbs, remaining cheddar cheese and parmesan cheese in medium bowl until mixed thoroughly. Sprinkle on top of pasta.
Bake until topping is golden brown and sauce is bubbly, about 30-35 minutes.
Calories: 612kcal, Carbohydrates: 53g, Protein: 25g, Fat: 33g, Saturated Fat: 20g, Trans Fat: 1g, Cholesterol: 93mg, Sodium: 838mg, Potassium: 350mg, Fiber: 2g, Sugar: 8g, Vitamin A: 1093IU, Vitamin C: 1mg, Calcium: 562mg, Iron: 2mg