Dissolve the salt in 4 litres of water in a large pot. Add the prawns and refrigerate 2 to 4 hours. Drain and pat prawns dry with paper towels.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling stir in the angel hair pasta and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through but is still firm to the bite, 4 to 5 minutes. Drain well in a colander set in the sink.
Meanwhile melt the butter and olive oil in a large frypan over medium-low heat. Stir in the garlic and cook until softened, 3 to 4 minutes. Add the prawns, white wine, lemon juice and chilli flakes. Cook and stir until the prawns are no longer translucent in the centre, about 6 minutes. Stir in the black pepper, parsley and lemon zest before tossing with the angel hair pasta.