Add a wire rack over a baking sheet, spray with cooking spray. Add the chicken, skin side up, on the rack. Sprinkle liberally with salt.
Place in the oven, then set the temperature to 300°F, set a timer for 25 minutes (allowing the chicken to cook in the oven while it preheats helps to render fat and make the skin crispier).
Add the cornstarch and beer to a saucepan, whisk until combined. Stir in the chili flakes, chili powder, sweet chili sauce, and sriracha.
Bring the mixture to a simmer over medium-high heat, stirring frequently until thickened, about 5 minutes.
After 25 minutes, remove the chicken from the oven, raise the heat to 400°F. Brush the chicken with glaze. Bake for 20 minutes.
Brush again with glaze, turn on the oven broiler.
Place the chicken under the broiler for 2-3 minutes or until the sauce starts to caramelize.
Remove from oven, add to a serving platter, sprinkle with green onions.