In a small mixing bowl, whisk together the soy sauce and cornstarch until no lumps remain. Add chicken stock, rice vinegar, brown sugar, and garlic and whisk until evenly combined.Put ramen in a large bowl and pour very hot tap water over it (I let the water run for a minute to get steaming hot). Set it aside to let the noodles soften.While noodles soak, toss chicken with some salt and pepper.Heat a wok or non-stick pan over medium-high heat. Add 1 Tbsp oil and then chicken. Saute until chicken is cooked through, 4 to 5 minutes. Set chicken aside.Return wok to medium-high heat. Add 1 Tbsp oil and then broccoli. Saute broccoli until it turns bright green and then continue cooking until broccoli is tender (if using frozen broccoli, just saute it until it's heated through and the excess water cooks off).Add sauce to the pan and toss to coat broccoli. Continue cooking until sauce reduces slightly, 1 to 2 minutes.Meanwhile, drain ramen noodles and add them to the pan. Add chicken to the pan and toss everything to coat in sauce.Remove form heat and add green onions and sesame seeds, if using. Serve immediately!