In a large pot of salted boiling water, cook pasta until al dente. Drain, reserving 1 cup pasta water, and return to pot.
Meanwhile, in a large skillet over medium heat, heat olive oil. Add shrimp and season with salt and pepper and cajun seasoning. Cook until pink, 2 minutes per side. Transfer to a plate.
Wipe out skillet and add butter.
Once melted, stir in flour and whisk until golden, 1 minute.
Add heavy cream and whisk until creamy, then add Parmesan and 1/2 cup pasta water. Whisk until creamy. Season with salt and pepper.
Return pasta to skillet and toss until creamy, then add shrimp and parsley and toss until combined.