Preheat the oven to 375°F (190°C). Line a baking sheet with foil and place a cooling rack on top; lightly spray the rack with nonstick cooking spray.
In a small saucepan off the heat, combine the sweet chili sauce, hoisin sauce, minced garlic, sesame oil, soy sauce, mandarin orange zest and juice, and salt and pepper; stir until well mixed.
Dip each chicken drumstick into the sauce to coat, then arrange the drumsticks on the prepared rack and reserve the saucepan with the remaining sauce.
Place the saucepan over medium heat and bring the leftover sauce to a gentle simmer, stirring occasionally, until slightly thickened and glossy.
Bake the chicken in the preheated oven for 40–50 minutes, basting generously with the simmered sauce 2–3 more times during cooking, until the internal temperature reaches 165°F (74°C) and the skin is caramelized.
Transfer the cooked drumsticks to a serving platter, sprinkle with sesame seeds and finely chopped parsley, and serve.