Heat 2 inches oil in a wok or heavy pot over medium heat to 350 degrees.
Mix yolk with water using fat wooden chopsticks. Add the flour all at once and mix just slightly. There should be large lumps of flour and a rim of flour dust around the bowl. Over mixing will result in a heavy batter coating. It is best to mix the batter in this amount, and repeat as necessary. Prepare ingredients, dry them, and dredge lightly in flour before dipping in batter.
When oil is hot, dip the vegetables one at a time into batter to coat completely, and then allow excess to drain into bowl. Working in batches, about 6 pieces at a time, and using a spider, carefully submerge vegetables in oil. Cook, turning once, until light golden, puffed and cooked through, about 3 minutes for most vegetables. They should be tender when pierced with the tip of a paring knife. Transfer to a baking sheet fitted with a wire rack to drain. Serve with dipping sauce. To serve sweet potato tempura for dessert, sprinkle with sugar and serve immediately.