Measure out all sauce ingredients.
To make the sauce: Melt butter over medium heat in a saucepan. Then add flour and whisk constantly for 2 minutes, to cook.
Slowly add the milk, whisking constantly. Cook the sauce, stirring frequently, for 10 minutes to thicken.
While the sauce is cooking, use butter to grease a 9x13 baking dish. Heat the water for the pasta and cook for 2 minutes less than directions on the box. Rinse in cold water and set aside.
After cooking the sauce for 10 minutes, remove from heat and add cheese, salt, chili powder, and garlic powder. Stir until cheese is melted and all ingredients are incorporated.
Place pasta in a large bowl and pour sauce right over it. Gently mix till covered.
Pour pasta into the buttered baking dish. Sprinkle with chili powder then cover with cheeses and cereal. Bake uncovered at 350° for 20 minutes. Let stand 5 minutes before serving.