Step 1: Sift together the flour, cornstarch, baking soda, baking powder, and salt into a large bowl to ensure a smooth, lump-free mixture.
Step 2: Gradually add the cold water to the dry ingredients, whisking continuously until a thick, smooth batter forms. Be careful not to overmix; a few small lumps are fine.
Step 3: Pat your fish fillets or chicken pieces dry with paper towels. Dip each piece into the prepared batter, ensuring it is evenly and thoroughly coated for the perfect crunch.
Step 4: Heat a generous amount of oil (enough to deep fry) in a heavy-bottomed pot or deep fryer to 350-375°F (175-190°C). Carefully place the coated fish or chicken pieces into the hot oil, frying in batches to avoid overcrowding.
Step 5: Deep fry the coated pieces for 4-7 minutes (depending on thickness) until they are golden brown, exceptionally crispy on the outside, and tender and juicy on the inside. Remove with a slotted spoon and place on a wire rack over paper towels to drain excess oil.