Bring a large pot of salted water to a boil. Cook the pasta according to the package directions until just tender. Before draining, scoop out and reserve 1 cup (240 ml) of the pasta water. Drain and set aside.

While the pasta cooks, pat the shrimp dry and season with 1/2 teaspoon of the salt, the pepper, and the red pepper flakes if using.

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

Reduce the heat to medium. Add the remaining 2 tablespoons of butter, then the garlic. Cook for 30 to 60 seconds, stirring often, until fragrant. Do not let the garlic brown.

Pour in the wine or broth and scrape up any browned bits. Simmer for 2 minutes, until slightly reduced.

Add the pasta and toss to coat, adding splashes of the reserved pasta water as needed to make a light, glossy sauce. Stir in the lemon juice and the remaining 1/2 teaspoon of salt.

Return the shrimp and any juices to the pan. Toss once more, top with parsley, lemon zest, and Parmesan if using, and serve immediately.