Whisk buttermilk and hot sauce in a bowl. Add shrimp and chill 10–15 minutes.
In a shallow dish, combine flour, cornstarch, baking powder, Old Bay, garlic, paprika, onion, cayenne, salt, and pepper.
Pour oil into a heavy pot to about 1½ inches. Heat to 360°F over medium-high heat.
Lift shrimp from buttermilk, letting excess drip off. Dredge in the seasoned flour, pressing to coat.
Fry shrimp in batches, 2–3 minutes, turning once, until golden and crisp. Do not crowd the pot.
Drain on a rack or paper towels. Season lightly with extra salt if desired.
Garnish with parsley. Serve hot with lemon wedges or cocktail sauce.