Cook the sushi rice and, while still warm, mix in 1 tbsp sugar and 1 tsp salt dissolved in the rice vinegar; leave to cool to lukewarm.
Break the eggs into a bowl, add the sake, 1 tsp sugar and a pinch of salt and beat until smooth but not frothy.
Mix the cornflour into the cold water and stir into the egg.
Heat a frying pan (oiled if not non-stick) over medium heat, pour in 3-4 tbsp egg and swirl to spread thinly.
When the edges start to dry, take it off the heat, flip after 30 seconds and return to the heat for 30 seconds; cook all the thin omelettes this way and let them cool.
Shape the rice into oblong portions, put one in the centre of each egg sheet and fold the edges over like a parcel.