Grease a shallow 30x25 cm rectangular tin. Preheat oven 170 degrees celcius.
Cream butter and sugar until light and fluffy, leave aside. Sift together the flour and baking powder and leave aside.
Beat egg whites until soft peaks form. Add in the flour/baking powder, mix until just combined. Next the zest and walnuts, mix then the butter sugar mixture and lightly mix again.
Pour into prepared tin and cook for around 10 minutes or until just undercooked. While cake is in the oven mix the middle part.
Beat together the egg yolks and sugar until light in colour and creamy. Spread onto the just undercooked cake and place back in oven for another 5 minutes or until dry.
Allow to cool completely before applying the top layer. Beat together butter and icing sugar until light and creamy. Place walnuts in a cup and add hot water.
Stir to combine and add to butter mixture. Beat until combined. Spread onto cooled cake.
