Add the chicken broth, tomato paste, smoked paprika, kosher salt, cracked pepper, and cayenne to a medium-sized mixing bowl. Whisk to combine.
Whisk in the Diet Coke.
Add the chicken thighs to a 10 to 12-inch deep, nonstick skillet over medium-high heat.
Pour the Diet Coke mixture over the chicken. Cover with a lid, and cook for 20 minutes, turning the chicken every 7 to 8 minutes. After 20 minutes, cook uncovered for another 8 to 10 minutes until the sauce thickens. Remove from the heat, garnish with parsley, and serve while hot.
Notes
You can also cook the dish in the slow cooker on high for 4 hours or 6 hours on low.
When cooking the chicken, ensure that the chicken's internal temperature reaches 165°F on a meat thermometer.
If you want your sauce to be thicker, you can stir a cornstarch slurry into the sauce before serving.