Make the dipping sauce (optional): In a small pan, combine the vinegar, ketchup, sugar, soy sauce, and water or pineapple juice. Bring to a simmer, then add the cornstarch slurry. Cook for 1–2 minutes, until the sauce thickens and becomes glossy. Set aside to cool.
Prepare the shrimp: Peel and devein the shrimp, keeping the tails on. If they’re tightly curled, make a few shallow cuts along the inside curve, then gently push along the back to straighten them so they cook evenly. Pat dry with paper towels so the batter sticks. Season with salt and any spices you like.
Make the batter: In a mixing bowl, whisk together the dry ingredients: flour, cornstarch, baking powder, and salt. In a separate bowl, whisk the wet ingredients: very cold water (you can add ice cubes), neutral oil, and egg white. Pour the wet ingredients into the dry ingredients and stir gently until just combined—don’t overmix. The batter should be thick but pourable; add a splash of cold water if needed.
Coat and fry the shrimp: Heat oil (2–3 inches deep) in a small saucepan to 350°F (bubbles form around a wooden chopstick when ready). Dip shrimp in batter, let excess drip off, and fry in batches until golden and crispy, about 6 minutes, flipping halfway. Drain on a rack or paper towels.
Serve: Enjoy the shrimp hot with your homemade sauce or any dipping sauce you like. They’re best while still crispy!