Add water and a bit of salt to a large pot and place over high heat for the pasta. While it's heating, prepare the cheese sauce.
In a large saucepan, melt the butter over medium heat. Whisk in the flour and continue to whisk and cook for about 2 minutes.
Very slowly add the milk, a little at a time, whisking constantly. Once all the milk has been added, cook, stirring frequently until the sauce thickens, about 10 minutes.
Remove the saucepan from the heat. Add the cheese, salt, ½ tsp of chili powder and garlic powder. Stir until the cheese is melted and all the ingredients are incorporated. Set aside for a minute.
Preheat oven to 350º. Oil or butter an 8-inch square baking dish.
When the pasta water is boiling, add the macaroni and cook for 1 minute less than the package directions indicate. When the pasta is cooked, drain and add to the cheese sauce. Mix gently, but thoroughly.
Spoon the mixture into a prepared baking dish. Sprinkle the top with the chili powder.
Bake uncovered for 20-25 minutes, until the sauce has bubbled up around the edges and the top has a nice golden crust. Let sit for a few minutes before serving.