Pat the chicken dry with paper towels. Place breast side up on a wire rack over a rimmed tray.
Mix the baking powder and kosher salt. Sprinkle a thin, even layer over the skin, including the legs and wings. Avoid thick patches.
Refrigerate the chicken uncovered on the rack for 12 to 24 hours, at 40°F (4°C) or colder. Keep it away from ready-to-eat foods.
Heat the oven to 425°F (220°C). Transfer the chicken and rack to a shallow roasting pan. Roast, checking after 55 minutes, until a thermometer registers 165°F (74°C) in both the thickest breast and thigh without touching bone. Tent loosely with foil if the skin browns too quickly.
Rest for 15 minutes before carving.