Preheat the oven to 425°F (218°C). Pat the 3 pounds of boneless, skinless chicken thighs dry with paper towels and set aside.
Make the sauce: combine 1/2 cup brown sugar, 8 cloves minced garlic, 2 tablespoons Sriracha, 1/4 cup soy sauce, 1/4 cup chicken broth, and 1/2 teaspoon black pepper in a saucepan over medium heat.
Whisk the sauce over medium heat until the brown sugar dissolves and the mixture is smooth, about 2–3 minutes; remove from heat.
Pour about half of the sauce into a 9×13-inch casserole dish and spread to coat the bottom. Arrange the chicken thighs in a single layer in the dish.
Pour the remaining sauce evenly over the chicken thighs.
Bake uncovered in the preheated oven for 35–40 minutes, basting the chicken with the pan sauce every 10 minutes, until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and let rest a few minutes before serving; garnish with chopped parsley or cilantro if desired.