Cook pasta in a large pot of salted water until al dente (usually 1–2 minutes less than the package says). Before draining, scoop out 1 cup of the starchy pasta water and set it aside. Drain the pasta and set aside.
Pat the Shrimp dry to ensure proper cooking.
In a very wide skillet, melt 2 tablespoon of the butter with the olive oil over medium-high heat. Add the shrimp in a single layer. Cook for about 90 seconds until most of the shrimp are just turning pink.
Add minced garlic and red pepper flakes if using. Sauté for about 60 seconds until fragrant.
Reduce heat to medium. Pour in your lemon juice, lemon zest, and stock. Add about half a cup of that reserved pasta water. Let it simmer and bubble for 2 minutes to reduce slightly and thicken.
Mix in chopped parsley, salt, and pepper, and check the seasoning.
If the sauce is too watery for your liking, stir in the remaining 2 tablespoons of cubed cold butter.
Add cooked pasta to the pot, toss everything together for 1–2 minutes over medium heat. If it looks too dry, add a splash more pasta water.
Serve garnished with more parsley, red pepper flakes, and parmesan cheese if desired.