Bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente according to package directions. Drain, reserve 1 cup of pasta water, and set aside.
In a large saucepan over medium heat, melt the butter. Slowly add the milk, then season it with salt, pepper, and garlic powder. Stir to warm the milk for 5 minutes.
Transfer the reserved cooked pasta on top of the butter and milk mixture. Add the reserved pasta water and stir until the sauce clings to the pasta.
Turn off the heat, add the grated cheeses and stir until the sauce has thickened and well combined with the pasta, about 5 more minutes. Serve immediately.
Notes
Storage: The pasta can be stored in an airtight container in the fridge for up to 3 days. You can reheat it on the stovetop (adding a little water to help loosen the sauce) or in the microwave until the pasta's warmed through.