Bring a large pot of salted water to a rolling boil over high heat. Add spaghetti or fettuccine and cook according to package instructions until al dente, about 8-10 minutes. Drain and set aside.

In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Season shrimp with salt and pepper, add to the skillet, and cook for 2-3 minutes until pink and opaque. Transfer shrimp to a plate and keep warm.

In the same skillet, add remaining butter, minced garlic, and red pepper flakes. Sauté for 1-2 minutes until fragrant and golden, being careful not to burn.

Return cooked shrimp to the skillet and add drained pasta, lemon juice, and half of the parsley. Toss gently for 1-2 minutes until well coated.

Adjust seasoning with salt and pepper as needed. Transfer to a serving dish, garnish with remaining parsley and grated Parmesan. Serve immediately.