Bring a large pot of generously salted water to a boil. Cook the 12 oz fettuccine until al dente, 10 to 11 minutes. Reserve ½ cup pasta water, then drain.

Melt the 4 tbsp unsalted butter in a large skillet over medium-low heat. Add the 3 cloves minced garlic and the ¼ tsp garlic powder and stir for 1 minute until fragrant.

Pour in the 2 cups heavy cream and raise the heat to medium. Simmer for 4 to 5 minutes, stirring occasionally, until the cream thickens slightly and coats the back of a spoon.

Reduce heat to low. Add the 1½ cups freshly grated Parmesan in 2 to 3 handfuls, stirring between each addition. Add the ½ tsp salt and ¼ tsp black pepper and stir for 1 minute until the sauce is smooth and glossy.

Add the drained fettuccine to the skillet and toss with tongs over low heat for 1 minute, adding reserved pasta water as needed to loosen the sauce. Divide into warmed bowls, crack extra black pepper on top, and finish with the 2 tbsp chopped fresh parsley.