Bring a pot of salted water to a boil. Cook the fettuccine or spaghetti according to package instructions until al dente. Reserve 1/2 cup of pasta water, then drain the pasta and set aside.
In a large skillet over medium heat, melt 2 tablespoons of butter. Add the diced chicken breast and season with salt and pepper. Cook until golden brown and fully cooked, about 6-8 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, add the remaining 2 tablespoons of butter. Once melted, add the minced garlic and sauté for about 1 minute until fragrant.
Add the cherry tomatoes and cook for another 2-3 minutes, until they begin to burst and soften.
Stir in the spinach leaves and red pepper flakes, cooking until the spinach is wilted.
Return the chicken to the skillet and mix all ingredients well. Add the drained pasta and a splash of the reserved pasta water to help bring everything together. Toss gently to combine.
Stir in the grated Parmesan cheese and adjust seasoning with salt and pepper if needed. If the pasta seems dry, add more reserved pasta water until the desired creaminess is achieved.
Serve immediately, garnished with fresh basil leaves and additional Parmesan cheese if desired.