Bring a large pot of salted water to a boil and cook pasta according to package directions. Reserve 1/2 cup pasta water before draining.
While pasta cooks, season chicken breasts with salt, pepper, and garlic powder on both sides.
Heat olive oil in a large skillet over medium-high heat. Add chicken and cook for 6-7 minutes per side until golden brown and cooked through to 165°F. Remove to a cutting board and let rest.
In the same skillet over medium heat, combine the pesto, heavy cream, and 1/4 cup of the reserved pasta water, stirring until smooth.
Slice the chicken into strips and add back to the skillet along with the drained pasta.
Toss everything together, adding more pasta water as needed to create a silky sauce that coats the pasta.
Stir in Parmesan cheese and any optional vegetables like spinach or tomatoes, cooking just until spinach wilts if using.
Serve immediately with extra Parmesan cheese on top.