Cook water chestnuts with 500ml water, custard powder and sugar. Stir until custard and sugar are dissolved.
Mix water chestnut flour with remaining water. Add cooked water chestnut mixture and mix well.
Transfer mixture to a 23 x 23-cm baking tin. Steam over rapidly boiling water for 25 minutes. Allow to cool and cut into 30–40 strips.
Combine batter ingredients except oil, adding enough water so batter is of a thin consistency and pours in an even flow. Leave batter to rest for 30 minutes. Test the consistency again and add more water if necessary. Stir in oil and use batter immediately.
Heat oil for deep-frying. Dip strips of water chestnut cake into batter and deep-fry until crisp. Remove and drain well.