In a small saucepan, combine the light soy, dark soy, sugar, sesame oil and water. Heat gently, stirring, until the sugar dissolves. Remove from the heat and set aside to cool.
In a medium bowl, whisk together the rice flour, cornflour, tapioca starch, water and salt until completely smooth. Rest the batter for 20 minutes.
Lightly oil a flat plate or shallow pan (about 15–20 cm wide) that will fit inside your steamer. Pour in a thin layer of batter and steam over high heat for 2–3 minutes, until just set.
Remove from the steamer, loosen the edges with an oiled spatula and gently roll into a log. If using prawns or char siu, scatter them over the sheet before rolling. Repeat with the remaining batter, oiling the plate each time.
Arrange the rice rolls on a serving plate, drizzle generously with the soy dressing and finish with spring onion. Serve warm.
