Bring a large pot of salted water to boil. Add the pasta and cook until al dente, about 8-10 minutes. Add broccoli florets in the last 3 minutes of cooking. Reserve 1/2 cup of pasta water, then drain and set aside.

In a mixing bowl, combine chicken with Cajun seasoning, garlic powder, and smoked paprika. Season with salt and pepper to taste and toss well.

Heat olive oil in a large skillet over medium-high heat. Add the seasoned chicken and cook for about 6-7 minutes, until golden brown and cooked through. Remove from skillet and set aside.

In the same skillet, melt the butter over medium heat. Sauté for 1-2 minutes until fragrant. Scrape brown bits for flavor.

Stir in crushed red pepper flakes (if using), Dijon mustard, lemon juice, and zest, and mix. Add parsley and chives, season with salt and pepper.

Return the pasta, broccoli, and chicken to the skillet. Toss together for about 2 minutes, adding reserved pasta water as needed for sauce consistency.

Stir in grated Parmesan cheese until melted and creamy. Serve warm, garnished with extra cheese and a squeeze of lemon juice.