Whisk brown sugar, water, fish sauce, rice vinegar, soy sauce, garlic, and ginger in a bowl until brown sugar is completely dissolved, about 1 minute. Set aside.
Heat oil over high heat in a skillet. Stir in chicken.
Pour 1/3 cup of the brown sugar mixture over chicken; cook and stir until the brown sugar mixture has a syrup-like consistency, 6 to 7 minutes.
Pour in the remaining brown sugar mixture; cook until chicken is tender and no longer pink inside, about 5 minutes.
Stir in in peanuts, jalapenos and green onion; cook until warmed through, 2 to 3 minutes.
