Bring a medium pot of salted water to a boil and cook the fettuccine according to package directions until al dente; reserve a small splash of pasta water if desired, then drain and set aside.
When the pasta is nearly done, melt the butter in a small saucepan over medium heat.
Stir in the half-and-half (or milk or cream), garlic powder, dried parsley (if using), a pinch of salt, a pinch of black pepper, and a pinch of cayenne (if using). Heat until the mixture is steaming but not boiling.
Reduce heat to low and stir in the shredded Parmesan until it melts and forms a smooth sauce; add a little reserved pasta water if the sauce seems too thick.
Toss the drained fettuccine with the sauce until evenly coated, then let sit for about 5 minutes to thicken slightly before serving.
Garnish with fresh parsley if desired and enjoy.