Remove chicken from the fridge 15 minutes before cooking. Slice each breast horizontally to create 4 fillets. Cover with plastic wrap and pound to an even 1/2-inch thickness. Pat dry with a paper towel.

Whisk all blackening seasonings together in a small bowl. Reserve 1 tablespoon for the sauce. Season both sides of the chicken fillets with the remaining seasoning and let rest.

Cook fettuccine in heavily salted boiling water until al dente, about 2 to 3 minutes less than the package directions. Reserve 1/2 cup pasta water before draining. Drain, rinse with cold water, and toss with a drizzle of olive oil. Set aside.

Melt 2 tablespoons butter with the olive oil in a large heavy-bottom skillet over medium-high heat. Once hot and sizzling, add chicken and cook 4 to 5 minutes per side until deeply golden and cooked through to 165 degrees F. Transfer to a cutting board, tent loosely with foil, and rest at least 5 minutes before slicing. Do not wipe out the skillet.

To the same skillet with drippings, melt remaining 2 tablespoons butter over medium-low heat. Add minced garlic and reserved 1 tablespoon blackening seasoning. Cook for 3 minutes stirring often to avoid burning the garlic.

Add half the chicken broth to the pan. Whisk remaining broth with cornstarch until smooth and pour into the skillet along with the heavy cream. Bring to a gentle simmer and cook until thickened, stirring occasionally, about 3 to 4 minutes. Reduce heat to low and stir in Parmesan until fully melted and silky, about 2 minutes. Add a splash of chicken broth if sauce becomes too thick.

Add cooked fettuccine and toss to coat, adding reserved pasta water a little at a time only if needed. Add sliced chicken and any accumulated juices from the cutting board. Season with salt, pepper, or cayenne to taste. Garnish with fresh parsley and serve immediately.