Boil your pasta according to package instructions, drain, and set aside
Meanwhile, melt your butter in a large skillet over medium high heat. When melted, whisk in your flour and let cook for about 30 seconds to 1 minute, whisking constantly
Add your milk, whisk to combine, and season with salt and pepper
Bring to a simmer and simmer until thickened (the cheese will thicken it further)
Add your cheese and garlic powder, stir to combine, and season to taste with salt and pepper