Prepare the penne according to package instructions. Drain and set aside.*
In a large cast-iron skillet, heat 2 tablespoons of vegetable oil on medium-high heat.
Butterfly the chicken breast and cut the seam to create 2 thinner slices, each about an inch thick. Season the chicken with desired amounts of salt and pepper.
Carefully lower the chicken into the preheated skillet and leave it untouched for several minutes while it gets a nice, crisp sear. This takes about 5-7 minutes or so.
Once the chicken begins to release a bit from the bottom of the pan, it’s ready to flip. Rotate the oil around in the pan to help it loosen if needed. Make sure it has a nice golden-brown color prior to flipping. You may need to decrease the heat slightly while the other side sears.
Once done, remove the chicken from the pan and set aside. Let it rest for 5-10 minutes before cutting it up. Leave the oil and chicken remnants in the pan as it will give the sauce added flavor.
In the same pan over medium heat, melt 2 Tablespoons of butter and use a spatula to release some of the browned bits from the bottom of the pan.
Add the diced onions and cook until they have softened and begin to caramelize. Add the garlic and cook for an additional minute.
Add the vodka to the pan and cook for 3-5 minutes.
Reduce the heat to low and add the diced tomatoes and marinara sauce, then stir in the heavy cream.
Add the cooked chicken back to the pan and stir into the sauce.
Sprinkle in the red pepper flakes and add the final Tablespoon of butter. Add in the pasta* (see notes) and use a silicone spatula to stir until the penne is evenly covered.
Top with Parmesan, chopped parsley and serve!
*If you plan on freezing leftovers, it's best to keep the sauce and pasta separated so that you can freeze the leftover sauce for a later meal, it reheats better than freezing the pasta and the sauce together. I like to make fresh pasta to go with the sauce that I froze. -To save time, I like to boil the pasta while the chicken sears.