In a large bowl, combine the soy sauce, vinegar, 2 tablespoons of the sugar and the egg whites. Stir until the sugar dissolves. Stir in the chicken.

Meanwhile, set a wire rack in a rimmed baking sheet. In a large bowl, mix the remaining 2 tablespoons sugar, the cornstarch, Sichuan pepper and 2 teaspoons salt.

Drain the chicken in a colander. Add a third of the chicken to the cornstarch mixture and toss to coat, pressing the pieces into the cornstarch. Transfer to a mesh strainer and shake to remove excess cornstarch. Transfer to the prepared rack in a single layer. Repeat, working in two batches, with the remaining chicken and cornstarch mixture.

Set another wire rack in a rimmed baking sheet. In a large Dutch oven, heat the peanut oil to 350°F. Add half of the coated chicken and cook, stirring occasionally, until well browned, about 5 minutes. Using a slotted spoon, transfer to the second rack. Allow the oil to return to 350°F, then repeat with the remaining chicken.

In a small microwave-safe bowl or glass measuring cup, microwave the Sichuan chili oil on high until just warm, about 30 seconds. Combine the chicken and scallions in a large bowl, sprinkle with the seasoning and drizzle with the warm chili oil, then toss to coat. Add the cilantro and toss again, then transfer to a platter. Serve with additional chili oil and seasoning salt at the table.