Heat a large pot over medium-high heat and add 1 tablespoon of butter.
Add the cubed chicken to the pot and season with onion powder, garlic powder, paprika, salt, pepper, and chili flakes.
Cook the chicken, stirring occasionally, until cooked through about 5 minutes; toss with 3 tablespoons of the basil pesto, then remove the chicken from the pot and set aside.
Add the remaining 1 tablespoon of butter and the chopped garlic to the same pot and cook until fragrant and lightly golden, about 1 minute.
Add the pasta and 2 cups of water to the pot. Bring to a simmer and cook, stirring often, until the pasta is al dente, about 8 minutes (adjust time per pasta package if needed).
Stir in the milk, cream cheese, Caesar dressing, the remaining pesto, and grated Parmesan; cook and stir until the sauce is smooth and creamy.
Return the cooked chicken to the pot, add the arugula or baby spinach and lemon juice, and stir until the greens wilt and everything is evenly combined.
Remove from heat and serve immediately, topping with extra Parmesan and fresh basil if desired.
