There are two secrets to making chicken stir-fry crispy. The first is how you dredge and fry it. You want a light, crispy batter that contains cornstarch or potato starch and all-purpose flour. The second secret is the sauce has to have a good amount of sugar to make a syrup. A syrup won't make crispy batter soggy.
Technique tip: If you can't find tempura flour, just combine 1 cup of all-purpose flour, 1 cup of cornstarch and 1 tablespoon baking soda.
Swap option: Feel free to substitute another meat or protein for the chicken.
For the sauce:
Add all the sauce ingredients to a 1-quart saucepan over medium heat. Whisk gently as it comes to a simmer. Allow to simmer, whisking constantly, for about 5 minutes, or until the sauce thickens. Remove from the heat and set aside.
For the chicken:1.
Heat 1 quart of the oil in a 4-quart Dutch oven to 165°F using a frying thermometer. Rinse the chicken under cold water and pat dry with paper towels.2.
In a medium-size bowl, combine the water and 1½ cups of the tempura flour into a thick batter; it should look like a thick pancake batter. Spread the remaining 1 cup of tempura flour on a plate. Dredge the chicken cubes in the tempura flour, coat in a thin layer of the batter, knock off the excess batter and fry in two batches until golden brown and crispy, 6-8 minutes. Drain on paper towels or a rack.3.
Heat the remaining 2 tablespoons of oil in a wok or large skillet over high heat. When you see the first wisps of white smoke, stir in the fried chicken, onion and scallions and cook for about 30 seconds. Stir in the sauce and allow to coat and simmer for a minute or two. Cook, folding with a spatula, until all the ingredients are well coated, about 2 minutes.