Sieve the flour, custard powder, baking powder and cooking soda.
In a big bowl mix together all the ingredients (condensed milk, flour, custard powder, baking powder, cooking soda, vanilla essence and butter) except the water, which has to be added gradually while beating the ingredients in the bowl.
Beat the mixture in the bowl for 20 mins.
Pre-heat the oven.
Grease a medium-sized square tin.
Dust with flour.
Evenly pour the cake mixture into it.
Bake at 150 degrees centigrade till done (approximately 30 mins).
Insert a knife in the centre of the cake to ensure that the cake is done.
Cool completely and remove from tin.
Cut into squares.
Serve plain with tea or with custard sauce as a dessert.