Cook the pasta. Bring a large pot of salted water to a boil. Add the pasta shells and cook according to package directions until al dente. Reserve about 1/2 cup of pasta water, then drain and set aside.
Season the chicken. Place the chicken pieces in a bowl and season with smoked paprika, garlic powder, onion powder, 1/2 teaspoon salt, and black pepper. Toss until evenly coated.
Cook the chicken. Heat the olive oil in a large skillet over medium-high heat. Add the seasoned chicken in a single layer and cook for 5–7 minutes, stirring occasionally, until golden brown and cooked through. Transfer to a plate.
Start the cheese sauce. Reduce the heat to medium and melt the butter in the same skillet. Add the minced garlic and cook for about 30 seconds, just until fragrant.
Make the roux. Sprinkle in the flour and whisk continuously for about 1 minute to remove the raw flour taste.
Add the milk and cream. Gradually pour in the warm milk while whisking constantly to prevent lumps. Stir in the heavy cream and remaining 1/4 teaspoon salt. Cook for 3–5 minutes, stirring frequently, until slightly thickened.
Add the cheese. Reduce the heat to low. Gradually stir in the shredded cheddar and Parmesan until completely melted and smooth. Add cayenne pepper if you prefer a little extra heat.
Combine everything. Return the cooked chicken to the skillet and add the drained pasta shells. Gently stir until everything is thoroughly coated in the creamy cheese sauce. If needed, add reserved pasta water a tablespoon at a time to reach your preferred consistency.
Serve. Remove from the heat and garnish with fresh parsley, extra Parmesan, or a light sprinkle of smoked paprika. Serve immediately while hot and creamy.