Cook pasta as per package instructions, drain, and set aside.
In a bag, mix 3 tsp Cajun seasoning, 1/4 tsp salt, and 1/4 tsp pepper. Add chicken pieces, seal, and shake to coat.
In a skillet, melt 2 tbsp butter over medium heat. Cook chicken until browned and cooked through. Set aside.
Melt remaining butter in the skillet. Add red onion and green bell pepper. Cook until softened, about 3-5 minutes.
Add heavy cream, smoked paprika, dried basil, garlic powder, remaining Cajun seasoning, 1/4 tsp salt, and 1/4 tsp pepper. Bring to a boil, then simmer until thickened.
Return chicken to skillet. Add Parmesan and pasta. Stir until cheese melts and sauce coats everything.
Serve warm.