Combine soy sauce, honey and ginger in a jug. Separate leaves from choy sum stems.
Place a large steamer over a wok or large saucepan of simmering water. Place choy sum stems and broccolini in steamer. Cook, covered, for 3 to 4 minutes or until just tender. Add choy sum leaves. Cook, covered, for 1 to 2 minutes or until wilted. Transfer to a bowl. Add sauce mixture. Toss to combine. Sprinkle with sesame seeds. Serve.