Preheat your oven to 400°F (200°C). Line a large, rimmed baking sheet with parchment paper or aluminum foil for easy cleanup. If using foil, lightly spray with non-stick cooking spray.

Place the chicken thighs on a cutting board. Using paper towels, carefully pat the chicken thighs thoroughly dry. Removing excess moisture is crucial for achieving a nice, crisp exterior crust.

In a small mixing bowl, combine all the ingredients for the ranch seasoning rub: the full packet of dry Ranch seasoning mix, garlic powder, onion powder, black pepper, and the optional Parmesan cheese. Stir until uniformly mixed.

Place the dried chicken thighs into a large bowl. Drizzle the 2 tablespoons of olive oil (or melted butter) over the chicken and toss lightly to ensure the thighs are evenly coated in fat. Sprinkle the prepared Ranch seasoning rub generously over the chicken pieces, using your hands or tongs to coat every surface thoroughly.

Arrange the coated chicken thighs on the prepared baking sheet in a single layer, ensuring they are not touching each other. Place the sheet into the preheated 400°F oven.

Bake for 25 to 30 minutes. The chicken is fully cooked when the internal temperature reaches 165°F (74°C) as measured by an instant-read meat thermometer inserted into the thickest part of the thigh.

Remove the baking sheet from the oven. Allow the chicken thighs to rest on the sheet for 5 to 10 minutes before serving. Garnish with fresh chopped parsley, if desired, and serve immediately.