To prepare the chicken, using a sharp knife, score the chicken breasts in a crisscross pattern without cutting through, then season the chicken with the salt and white pepper and coat lightly with the cornstarch, shaking off the excess.
To make the sauce, in a bowl, mix the chicken stock, soy sauce, sugar, mirin, and sake until the sugar is dissolved.
To pan-fry, in a pan over high heat, add the cooking oil, place the chicken crisscross side down, and cook until lightly golden on both sides, about 3 minutes total, then add the garlic and cook until fragrant.
To simmer, reduce the heat to medium-low, pour in the sauce, cover with a lid, and cook for about 10 minutes, flipping the chicken occasionally so it cooks evenly.
To reduce the sauce, remove the lid, increase the heat to high, and cook until the sauce thickens slightly and turns glossy, about 2 minutes, making sure the chicken reaches 165°F (74°C).
To serve, slice the chicken if desired and serve hot with the sauce spooned over the top.