Bring a large stock pot of water to a boil. Add 2 tablespoons kosher salt and dried angel hair pasta. Cook according to package directions, or until desired tenderness. Reserve 1 cup of cooking liquid.
In a large skillet over medium low heat, add the butter and heat until melted. Add the lemon juice, most of the lemon zest (reserve some for serving), 1/2 tsp salt, black pepper, and crushed red pepper flakes (if using). Stir until combined.
Transfer cooked pasta with tongs or a spider to the butter mixture. Add 1/2 cup of pasta water and toss to combine. If it's too dry, or you'd like it saucier, add the remaining 1/2 cup of water. The pasta will continue to absorb the liquid, so the saucier to start, the better.
Sprinkle with minced parsley and remaining lemon zest, divide and serve immediately.