TO prepare the custard, combine milk, cornflour, custard powder, sugar and salt in a saucepan. Stir in hot water and mix well. Cook the mixture over a low heat, stirring continuously until it thickens.
Remove saucepan from heat and add butter and puréed sweet corn paste. Keep stirring until well mixed. Dish out and set aside to cool completely.
Refrigerate before use.
To prepare glutinous rice ball dough, put wheat starch in a bowl and pour in boiling water. Stir well with a pair of chopsticks until a soft dough is formed.
Combine glutinous rice flour and the cooked dough in a large mixing bowl. Stir in water and knead to form a soft dough. Divide dough into small balls. Flatten each ball into a thin disk. Spoon a little custard filling in the centre and wrap up to secure the filling.
Repeat with the rest of the dough. Bring a pot of water to the boil; drop in the prepared balls of dough. When the balls float, remove with a slotted spoon and place in cold water for 10 minutes. Serve tong yuen in hot heavy ginger syrup.
To prepare ginger syrup, grate ginger and set aside. Bring water to the boil with the three types of sugar. Put in the grated ginger and simmer for 15 minutes. Strain the syrup and set over a gentle low heat to heat up.