Combine the chicken, soy sauce, white pepper, egg white and 3 teaspoons of the cornstarch in a small bowl; stir until the mixture clumps together.
Use a fork to whisk together the remaining teaspoon of cornstarch and the water in a liquid measuring cup to form a paste. Add the garlic, ginger, sugar, vinegar and chicken broth; mix well to form a sauce. Taste and add sugar and/or vinegar as needed.
Heat enough of the oil to measure ½ inch deep in a large skillet or wok over medium-high heat until the oil shimmers. Line a plate with a few layers of paper towels.
Add the chicken to the skillet or wok in a single layer, then stir-fry immediately a few times, scraping it from the bottom to keep the chicken from sticking. Cook for 2 or 3 minutes or until the chicken is golden and crisped. Use a slotted spoon or spatula to transfer it to the paper-towel-lined plate to drain.
Pour out all but 1 teaspoon of the oil, then return the skillet or wok to medium-high heat.
Add the scallion and crushed red pepper flakes. Stir-fry for 1 minute, then add the sauce and cook for 1 to 2 minutes, until the mixture thickens.
Return the chicken to skillet or wok, stirring to heat through and coat with the sauce. Eat hot.