Mix buttermilk with 1 teaspoon salt, 1/2 teaspoon pepper and 1 teaspoon paprika in a covered nonreactive container. Add chicken and refrigerate at least 4 hours or up to 12 hours. Do not marinate at room temperature. Set a clean wire rack over a tray for cooked chicken.

Mix flour, cornstarch, baking powder, remaining salt, pepper and paprika, garlic powder, onion powder and optional cayenne in a wide dish. Lift chicken from buttermilk, let excess drip, then press firmly into the flour mixture. Set coated chicken on a separate rack for 10–15 minutes while oil heats. Discard raw marinade and unused contaminated flour.

Heat 1 1/2 inches oil in a heavy deep skillet or Dutch oven to 325°F (163°C), using a frying thermometer. Keep the pan no more than half full and lower pieces carefully away from you. Fry in batches without crowding, adjusting heat to hold roughly 300–325°F (150–163°C).

Turn occasionally and fry about 15–20 minutes per batch, until the crust is deeply golden and every piece reaches at least 165°F (74°C) in the thickest meat without touching bone. Large thighs may take longer; do not serve based on crust color alone.

If the crust is ready before the center, transfer to a clean rack on a baking sheet and finish in a 350°F (175°C) oven until 165°F. Drain cooked chicken on the clean rack, not on the raw-chicken tray, and rest 5–10 minutes before serving. The 45-minute cooking allowance assumes two batches; add time for a smaller pan.