Combine the chicken, citrus juice, soy sauce, garlic, brown sugar and black pepper and refrigerate for 2 to 6 hours.
Drain the chicken well and pat the surface lightly so the sugary marinade does not drip into the oil.
Whisk the flour, cornstarch and salt in a shallow pan, then press each piece into the mixture once for a thin complete coat.
Rest the chicken on a rack for 15 minutes while heating 1 1/2 inches oil to 325°F.
Fry in two batches for 14 to 17 minutes, turning every 3 minutes and keeping the oil between 310°F and 325°F.
Remove each piece when dark golden and at least 165°F beside the bone, then drain on a clean rack.
Rest for 8 minutes before serving; the citrus aroma is clearer once the chicken is not screaming hot.