Preheat the oven to 375°F (190°C). Line a baking sheet with foil and set a cooling rack on top; lightly spray the rack with nonstick cooking spray.
In a small bowl or off the heat in a small saucepan, combine the sweet chili sauce, hoisin sauce, minced garlic, sesame oil, soy sauce, mandarin zest and juice, and a pinch of salt and pepper; stir until well blended.
Dip each drumstick into the sauce to coat, then place the coated drumsticks on the prepared rack on the baking sheet.
Place the saucepan over medium heat and bring the remaining sauce to a simmer, cooking briefly until it thickens slightly and becomes glossy (about 2–4 minutes).
Bake the drumsticks for 40–50 minutes, basting generously with the simmered sauce 2–3 more times during baking, until an instant-read thermometer inserted into the thickest part registers 165°F (74°C).
Remove the drumsticks from the oven and sprinkle with sesame seeds and finely chopped parsley, then serve.
