In a large bowl, combine the soy sauce, vinegar, 2 tablespoons of the sugar and the egg whites. Stir until the sugar dissolves. Add the chicken and stir to coat, then cover and let marinate at room temperature for 20 to 30 minutes.

Meanwhile, set a wire rack in a rimmed baking sheet. In a large bowl, stir together the remaining 2 tablespoons sugar, the cornstarch, Sichuan pepper and 1 teaspoon salt.

Drain the chicken in a colander. Add ⅓ of the chicken to the cornstarch mixture and toss to coat completely, pressing the pieces into the cornstarch to make it adhere.

Transfer the coated chicken to a mesh strainer and shake to remove excess cornstarch. Transfer to the prepared rack in a single layer. Repeat with the remaining chicken and cornstarch mixture.

Set a second wire rack in a rimmed baking sheet. In a large Dutch oven over medium-high, heat the peanut oil to 350°F. Add half of the coated chicken and cook, stirring occasionally to prevent sticking, until well browned, about 5 minutes. Using slotted spoon or wire skimmer, transfer the chicken 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 hot fried chicken and scallions in a large bowl, sprinkle with the Sichuan seasoning salt and drizzle with the warm chili oil, then toss to coat.

Add the cilantro and toss again, then transfer to a platter. Serve, passing additional chili oil and seasoning salt at the table.