How to Make It
To julienne the sweet potatoes, cut a sliver from one broad side of each sweet potato. Steady it on the flat side and begin slicing it as evenly as possible into very thin (about ⅛-inch) planks; it is more important that they be uniformly thin than any exact thickness. Stack the planks up, then slice them lengthwise into matchstick-size pieces. If necessary, cut them crosswise into 1-inch lengths.
Combine the sweet potatoes, leek, serrano, and shrimp in a large bowl and season with the salt. Add 1 to 2 tablespoons of the cake flour and toss; the flour will help the batter adhere to the ingredients.
In a separate medium bowl, whisk together the remaining cake flour, the cornstarch, and ¾ cup ice water. Add more water, a tablespoon at a time, if needed to make the batter smooth and fall like a ribbon. Pour the batter over the vegetables and shrimp, and fold until combined. The kakiage mixture should look like creamy, overdressed coleslaw.
Arrange several layers of brown paper or paper towels on a large rimmed baking sheet. In a large, heavy pot fitted with a deep-fry thermometer, heat 2 inches of oil over medium heat until it reaches 325°F; moderate the heat to maintain this temperature as you cook. Armed with two metal tablespoons, scoop up some kakiage mixture with one spoon and use the other to slowly slide the mixture into the oil. The first bits of batter to hit the oil will puff up and serve as a life preserver for the rest, keeping them afloat until the oil can work its crisping magic. Repeat until you have 4 or 5 floating vegetable webs, with about an inch of space around them. Bob and baste the fritters, turning once, until they are golden and crunchy all over, 2 to 2½ minutes.
Lift the fritters from the oil using a spider and lay them on the prepared baking sheet. Sprinkle with a tiny pinch of salt and repeat forming and frying fritters until all the mixture is used up. If the fritters become cold before they are all fried, reheat them briefly in a 350°F oven (do not hold them in the oven or they will become soggy).