Preheat oven at 180°C. Place the egg yolks in a large bowl and whisk well. Add lemon juice and lemon peel and mix. Add oil and mix. Add ¼ cup water and mix. Sieve refined flour, baking powder and salt into another bowl.
Place the egg whites into a clean bowl and beat with a clean beater till stiff. Add egg powder, if using, and beat again.Add castor sugar to the flour mixture and mix well.
Add egg yolk mixture to the flour mixture and mix well. Add the egg whites, little by little, mixing with each addition till everything is well incorporated.Pour the mixture into a rectangular silicon mould. Keep it in the preheated oven and bake for 30 minutes or till done.
Remove from the oven and set aside to cool. Remove from the mould and cut into slices.
Dust icing sugar over them and serve with tea or coffee.