Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone baking mat.
Cut the potatoes into 1-inch cubes and toss with olive oil, garlic powder, salt, and black pepper. Spread the potatoes on the prepared baking sheet and bake for 20-25 minutes.
Pat the chicken thighs dry and season with salt and black pepper.
In a small bowl, combine melted butter, minced garlic, Parmesan cheese, dried parsley, and red pepper flakes (if using).
Push the potatoes to the sides of the baking sheet and place the seasoned chicken thighs in the center. Pour the Parmesan garlic sauce over the chicken.
Bake for an additional 20-25 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F (75°C).
Optional: Broil the dish for 2-3 minutes to achieve a crispy top.
Serve the Garlic Parmesan Chicken and Potatoes hot, garnished with fresh parsley or chopped green onions.