Begin by preparing your dipping sauce. In a small bowl, combine vinegar, minced garlic, chopped chili, sugar, and a pinch of salt. Set this aside to let the flavors develop while you make the fritters.
Get your vegetables ready. Cut your buttercup squash and carrots into thin, matchstick pieces about 2 inches long. Slice your spring onions and red onion thinly. Pat all vegetables dry with a clean kitchen towel.
Make your batter by whisking together flour, cornstarch, eggs, cold water, fish sauce, minced garlic, salt, and pepper in a large bowl until smooth. The batter should slowly drop from a spoon - if it's too thick, add cold water one tablespoon at a time.
Add all your prepared vegetables to the batter. Gently mix everything until all vegetables are evenly coated.
Heat oil in a heavy pan until it reaches 350°F (175°C). If you don't have a thermometer, drop a tiny bit of batter - it should sizzle and float immediately.
Scoop ½ cup of the mixture for each fritter and carefully drop it into the hot oil. Quickly flatten it with your spatula until it's about ¼-inch thick.
Fry each fritter for 3-4 minutes until the bottom turns golden brown and crispy. Flip once and cook the other side for another 3-4 minutes.
Remove from the oil when both sides are crispy and golden brown. Place on paper towels to drain excess oil.
Clean out any loose bits from the oil with a strainer between batches. Wait for the oil to get back to temperature before adding the next batch.
Serve immediately while hot and crispy with your prepared dipping sauce. For the best experience, eat within 15 minutes of cooking when the ukoy is at its crunchiest.
Remember: Keeping your oil at the right temperature is key - not too hot, not too cool. You should hear a steady, gentle sizzle while frying.