Season chicken with salt and pepper. Toss lightly with cornstarch if using.
Heat oil in a large skillet over medium-high heat. Cook chicken in a single layer until golden and cooked through. Remove and set aside.
Lower heat to medium. Add garlic and ginger to the pan; cook until fragrant, about 30 seconds.
Whisk peanut butter, soy sauce, water, honey, and vinegar until smooth. Pour into the pan and stir.
Simmer gently until sauce thickens and becomes glossy.
Return chicken to the pan and toss until fully coated.
Cook 2–3 minutes until sauce clings to the chicken.
Garnish and serve warm.