Place a large non-stick frying pan or wok over high heat.
Add the oil and garlic. Saute for 2-3 minutes until fragrant.
Add the chicken, salt, and pepper. Saute until the chicken is browned, then add the broccolini and butter.
Cook for 5 minutes, until the broccolini has softened.
Add the pak choy and tamari sauce and saute until the greens are just cooked (and still a little crunchy).
Serve the stir fry sprinkled with sesame seeds.