First, prepare your shrimp. Remove the heads but keep the tails on. Cut a line down the back of each shrimp and remove the dark vein. Make three small cuts on the belly side to prevent curling. Pat the shrimp completely dry with paper towels.
Mix the cleaned shrimp with calamansi juice (or lemon), salt, and pepper in a bowl. Cover and refrigerate for 10 minutes.
While the shrimp marinates, sift together flour, cornstarch, baking powder, salt, and white pepper in a large bowl. Put this dry mixture in the refrigerator for 30 minutes - cold ingredients are the secret to crispy coating.
Heat your oil in a deep pot or wok. You'll need about 3 inches of oil, heated to 350°F. If you don't have a thermometer, test by dropping a small bit of batter - it should sizzle and float immediately.
Take your flour mixture out of the fridge. In a separate bowl, mix the cold egg with ice-cold water. Pour this into your chilled flour mixture and stir gently just until combined. Don't worry about small lumps - they're good for crispiness.
Put some cornstarch on a plate. Take each marinated shrimp, coat it in cornstarch, shake off the excess, then dip in your batter. Hold it by the tail and let extra batter drip off.
Carefully place each battered shrimp in the hot oil. Don't crowd the pot - fry only 4-5 pieces at a time. Cook for 2-3 minutes until golden brown.
Lift out the crispy shrimp with a strainer and place them on a wire rack, not paper towels - this keeps them crispy. Let the oil return to 350°F before cooking the next batch.
Serve your Camaron Rebosado immediately while hot and crispy. Arrange on a platter with sweet and sour sauce, banana ketchup, or sweet chili sauce for dipping.
Remember: these are best eaten right away while the coating is perfectly crispy and the shrimp is piping hot. If you need to keep them warm, place in a 200°F oven for no more than 30 minutes.