Pat chicken completely dry with paper towels
Mix buttermilk, soy sauce, and all marinade spices in large bowl
Add chicken, coat well, refrigerate 4+ hours (overnight best)
Whisk flour, cornstarch, and all breading spices in shallow dish
Remove chicken from marinade, let excess drip but keep moist
Dredge in breading, pressing firmly to coat. Shake off excess
Dip briefly back in marinade, then dredge again in breading
Place on wire rack, rest 15-20 minutes at room temperature
Heat oil to 325°F in deep fryer or large pot
Fry 2-3 pieces at a time, don't overcrowd
Fry 12-15 minutes for dark meat, 10-12 for white meat, until golden and 165°F internal
Remove to wire rack over baking sheet to drain
Rest 5 minutes before serving