Preheat the oven to 425 degrees F.
Heat a medium size pot on medium heat and add butter. Stir the butter around the pot until it's melted. Add flour and stir continuously.
When the butter and flour start to thicken (about 2 minutes), add milk and stir. Stir in garlic powder and salt. Remove the white sauce from the heat and set aside for a moment.
In a medium size casserole dish, add the cooked pasta. Pour the white sauce over the pasta and stir together to coat the pasta.
Add half of the cheese to the pasta and white sauce mixture. Stir so that the cheese is mixed in with the pasta and white sauce.
Top the pasta mixture with the remaining cheese. Bake in a preheated oven until the cheese melts (about 15 minutes).
Serve right away while warm.