Bring a large pot of salted water to a boil. Add pasta and cook according to package directions until al dente. Reserve 1 cup of pasta water before draining.

While pasta cooks, pat shrimp dry and season with salt and pepper. Melt 2 tbsp of butter in a large skillet over medium-high heat. Add shrimp in a single layer and cook for 1-2 minutes per side until pink. Remove from skillet and set aside.

In the same skillet, add the minced garlic and red pepper flakes. Cook for 30 seconds until fragrant.

Pour in the white wine or broth, scraping the bottom of the pan. Simmer for 1 minute. Reduce heat to low, add the remaining 4 tbsp of butter, lemon juice, and lemon zest. Whisk until smooth.

Add the drained pasta to the skillet, tossing to coat in the sauce. Add the cooked shrimp and fresh parsley. Toss to combine. If needed, add reserved pasta water to reach the desired sauce consistency.

Serve immediately, garnished with extra parsley and grated Parmesan cheese if desired.