Boil a large pot of salted water over high heat, add the 12 oz fettuccine, and cook 10 to 11 minutes until al dente. Reserve ½ cup pasta water, then drain.

Heat 1 tbsp olive oil and 1 tbsp butter in a large skillet over medium-high. Season the 1 lb shrimp with ¼ tsp garlic powder, ¼ tsp salt, and a pinch of pepper. Sear 1 to 2 minutes per side until pink and opaque, then transfer to a plate.

In the same skillet over medium heat, melt the remaining 2 tbsp butter. Add the 4 cloves minced garlic and stir for 1 minute. Pour in the 2 cups heavy cream and simmer gently for 3 to 4 minutes.

Reduce heat to low. Add the 1½ cups grated Parmesan in small handfuls, stirring after each addition. Season with the remaining ¼ tsp salt and ¼ tsp black pepper. Stir for 2 minutes until smooth and thick enough to coat a spoon.

Add the drained fettuccine to the skillet and toss with tongs over low heat for 1 to 2 minutes, adding reserved pasta water as needed. Return the shrimp to the pan, divide into 4 warm bowls, and scatter the 2 tbsp chopped parsley over each serving.