Bring a large pot of water to a boil. Add the 6 oz (180 g) buckwheat (soba) noodles and cook for about 3 minutes until just tender. Drain the noodles and rinse under cold water to stop cooking; drain well and set aside.
While the water is coming to a boil or while the noodles cook, prepare the other ingredients: ensure the 7–8 oz chicken breast is in thin slices; core and thinly slice (julienne) the yellow and red bell peppers; thinly slice the red onion; wash the carrot and use a vegetable peeler to shave it into ribbons; slice the 3 pieces of baby corn into bite-size pieces; thinly slice the chili pepper (remove seeds if you want less heat); if the 2 oz frozen green beans are still frozen, rinse briefly under cold water to separate and pat dry.
Heat a wok or large nonstick skillet over high heat until very hot. Add all 3 tsp sesame oil and swirl to coat the surface.
Add the sliced chicken in a single layer and stir-fry, tossing frequently, for about 2–3 minutes until the pieces are mostly cooked through and beginning to brown.
Add the prepared vegetables (baby corn, green beans, carrot ribbons, red onion, yellow and red bell peppers, and chili) to the wok. Stir-fry over high heat for 2–3 minutes until the vegetables are crisp-tender.
Add the drained noodles and all 6 tsp soy sauce to the wok. Toss everything together and stir-fry for another 1–2 minutes until the noodles are heated through and the soy sauce evenly coats the chicken and vegetables.
Remove from heat and serve immediately.