Holding the squid firmly, pull the head from the body including internal parts and tentacles. Pull out the clear cartilage inside the tube and discard.
With a knife, cut the head and the ink sack from the tentacles. Set aside the tentacles. Peel the spotted outer membrane from the squid and discard.
Under cold running water, wash the tentacles and the tubes inside and outside to remove any remaining tissue or debris. Drain well and pat dry.
Cut the body into rings by slicing vertically or keep them whole depending on size.
In a bowl, place milk and cleaned squid. Marinate in the refrigerator for at least 30 minutes or up to 4 hours. Drain squid and discard milk.
In another bowl, combine flour and cornstarch. Stir to combine.
Toss the squid in the flour mixture to fully coat.
In a wide pan or wok over medium heat, heat about 3-inches deep oil to 350 F.
Add coated squid in batches and deep-fry, turning as needed, for about 1 to 3 minutes or until crisp and golden.
With a slotted spoon, remove from pan and drain on a wire rack set over a baking sheet.
Season with garlic powder, salt, and pepper to taste. Serve immediately with lemon wedges or spicy vinegar.