Add ice water little by little. Sift dry ingredients together and fold in.
Beat egg whites, fold them carefully into the cake mixture.
Pour the batter into prepared 2 round cake tins.
Bake for 20 minutes at 180°C or until the test pin comes out clean.
For the Custard:
Mix the custard powder and milk. Bring to a boil. Let it cool slightly, add the butter and caster sugar with the vanilla into the custard, whisk it well, place in the fridge to cool, but remember to stir occasionally.
Assembling the cake:
Divide each cake into two slices and fill with custard between layers and on top. Add some of the leftover crumbs to sprinkle on top.