In a large pot of boiling salted water, cook the pasta according to package instructions; drain well and reserve 1 cup of pasta water.
In a large skillet over medium high heat, melt two tablespoons butter. Add shrimp, salt and crushed red pepper, and cook stirring occasionally, until shrimp is opaque pink, about 2-3 minutes. Remove from heat and set aside.
In the same skillet, add the remaining butter and garlic and cook until fragrant, about one minute. Add the lemon juice, pasta water and parmesan cheese and stir to combine. Transfer the spaghetti to the skillet and toss with the sauce.
Add the shrimp on top and garnish with parsley. Serve immediately.