Bring a large pot of salty water to boil.
Meanwhile, Transfer 300g shrimp to a plate and pat to dry with paper towel. Season all other with 1/2 tsp salt. Set aside. Thinly slice 6 garlic cloves. Chop enough parsley to fill 1/4 cup. Grate enough parmesan for 1/3 cup. Set aside.
Heat a large frying pan on medium-high heat. Add 1 tbsp olive oil to the pan. Once hot, add shrimp and sear until golden brown on one side and pink throughout. You might have to work in batches depending on the size of your pan. Remove cooked shrimp and transfer back to the plate.
By now, your water should be boiling. Drop 250g spaghetti and cook for 2 minutes less than the package instructs. We want it to be extra al dente.
Back in the pan, reduce heat to medium-low. Add 2 tbsp unsalted butter. Once foaming, add 1 tbsp crushed Calabrian peppers and sliced garlic. Cook until the garlic is soft but not brown, about 5-7 minutes. Add shrimp back to the pan and toss to coat. Turn heat to low until pasta is ready.
Reserve 1 cup pasta water and drain the rest. Transfer cooked spaghetti to the pan along with 1/2 cup pasta water and remaining 1 tbsp butter. Increase heat to medium and stir constantly until the sauce starts to thicken and emulsify, adding more pasta water as needed. Stir in 1/4 cup chopped parsley and 1/3 cup grated parmesan and serve immediately.
