Bring a medium saucepan of water to the boil for the noodles. Thickly slice the choy sum, keeping the stems and leaves separate. Halve the carrot lengthwise and thinly slice. Thinly slice the spring onion.
Step 2 — Make sauce
Combine the kecap manis, 1 tsp Chinese barbecue seasoning, a pinch of chilli flakes, if desired, 2 tsp white vinegar, 1 tsp sugar and 1 tsp soy sauce in a bowl.
Step 3 — Prep and cook chicken
Combine the remaining Chinese barbecue seasoning and 1 tbs oil in a large bowl. Put the chicken flat on a board, put your hand on top and halve horizontally into fillets. Add to the marinade, season with pepper and turn to coat. Heat a medium deep frypan over medium-high heat. Cook the chicken for 2-4 mins each side until cooked through. Remove from the pan and reserve the pan.
Step 4 — Cook noodles
Meanwhile, cook the noodles in the pan of boiling water for 8 mins or until tender. Drain, then rinse under cold water to prevent the noodles from sticking.
Step 5 — Start stir-fry
Heat 1 tbs oil in the reserved pan over high heat. Stir-fry the choy sum stems and carrot for 2 mins or until just tender. Add half the spring onion and stir-fry for 1 min.
Step 6 — Finish stir-fry and serve
Add the choy sum leaves, noodles and sauce to the vegetables and cook, stirring, for 1-2 mins until combined and heated through. Remove the pan from the heat. Slice the chicken. Divide the noodle stir-fry and chicken among bowls. Scatter with the remaining spring onion and serve with the remaining chilli flakes, if desired.