Cook fettuccine noodles according to package directions, reserving ½ cup pasta water, drain, and set aside.

While the pasta cooks, melt 1 tablespoon butter with 1 tablespoon oil in a large skillet over medium-high. Season the chicken breasts with 1 teaspoon kosher salt and black pepper. Cook the chicken breasts for 7 minutes, turn, and cook for another 5-7 minutes or until the chicken is browned and their juices run clear. Remove the chicken from the skillet, cover with foil, and set aside to rest.

In the same skillet, melt 1 tablespoon of butter. Add the leeks and cook for about 3 minutes or until they soften. Add the garlic, carrots, and celery. Season with the remaining kosher salt and cook for 2 minutes, stirring frequently.

Add the chicken stock, then bring the sauce to a boil while scraping the cooked flavor bits from the skillet. Reduce the heat to a rolling simmer and cook until the stock is reduced by half, leaving about 1 cup. Add the heavy cream and bring to a simmer. Reduce the heat to medium and cook for 5-7 minutes or until the sauce thickens. Stir in the buffalo sauce and Parmesan cheese, then cook for another minute. Remove the sauce from the heat and stir in the remaining 2 tablespoons of butter until it melts and the sauce is glossy.

While the sauce cooks, slice or shred the chicken into thin strips. Add the chicken and fettuccine noodles to the sauce and toss to coat. Cook for another 1-2 minutes and season with more salt if desired. If the sauce is too thick, add the reserved pasta water 1 tablespoon at a time until it’s the desired consistency. Garnish with parsley and serve with additional Parmesan cheese.