Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions until al dente. Reserve 60 ml (1/4 cup) of cooking water, then drain the pasta.

While the pasta cooks, pat the shrimp dry and season with salt and black pepper.

Heat olive oil in a large skillet over medium-high heat. Add the shrimp in a single layer and cook for 1-2 minutes per side, until just pink. Remove to a plate and set aside.

In the same skillet, add minced garlic and red pepper flakes (if using). Sauté for about 30 seconds, until fragrant.

Lower the heat. Add the lemon zest and juice, stirring to loosen any bits from the pan. Add the butter and let it melt into the sauce.

Return the shrimp to the skillet, add the drained pasta, and pour in the reserved pasta water. Toss everything together for 1-2 minutes, until well combined.

Off the heat, toss in the grated Parmesan cheese. Adjust seasoning with more salt, pepper, or lemon to taste.

Serve immediately, sprinkled with chopped parsley and extra Parmesan if desired.