In a medium bowl toss together the chicken and the cornstarch until the chicken is well coated. Toss again with 1/2 teaspoon salt and 1/4 teaspoon pepper. Set aside.
Heat a large skillet, preferably nonstick, over medium-high heat. Add the peanuts and toast them in the pan, stirring them frequently, until the smell nice and nutty and are golden brown, 1 to 2 minutes. Transfer them to a small bowl.
Spray the same skillet with nonstick spray if you are not using a nonstick skillet. Heat 1 tablespoon of the oil in the skillet over high heat, and sauté half of the chicken until it is lightly browned all over, 3 to 4 minutes, but not cooked through. Transfer the chicken to a plate, and repeat with another tablespoon oil and the rest of the chicken. Transfer the chicken to the plate with the first batch of chicken.
Return the pan to medium high heat, add the remaining teaspoon of oil to the pan, and then add the scallions and garlic and saute for 2 minutes, until they start to become tender. Add the vinegar to the pan and stir to deglaze, scraping up any flavorful bits that have stuck to the bottom of the pan, until the vinegar has pretty much evaporated, just one minute. Return the chicken to the pan, add the hoisin sauce and stir to coat well and cook through, about 2 minutes. Stir in the toasted peanuts and serve over the hot rice.