Boil the chicken breast and shred it along the grain.
Julienne the cucumber and carrot.
Slice the onion thinly, soak it in cold water, then drain.
Cook the buckwheat noodles and rinse thoroughly with cold water.
Mix gochujang (red chili paste), red pepper powder, soy sauce, sugar, minced garlic, vinegar, sesame oil, and spicy chicken sauce to make the seasoning sauce.
Add the seasoning sauce to the noodles and mix well, then top with the prepared vegetables and shredded chicken breast.
Sprinkle sesame seeds on top to finish.