25gdark chocolatekeep in freezer and grate into cake just before putting into pan
Preheat oven to 170C. In a mixing bowl beat together the sugar and butter until pale and fluffy.
In a separate bowl sift together the flour and baking powder.
Add the egg yolks to the creamed butter and sugar mixture. Next beat the dry ingredients alternately with the milk till all incorporated.
In a clean bowl whisk the egg whites until think and stiff peaks. Fold the egg whites gently into the cake mixture. Finally grate the chocolate into the cake mixture and fold in carefully so as not to melt the chocolate as you still want "flecks" to bake into the cake.
Pour batter into a tray bake pan (approx 9" x 13") and bake in centre rack at 170C for 20-25 min until skewer comes out clean.
With a mixer beat the butter until light and creamy, this could take up to 5 minutes. Gradually add the icing sugar and continue to beat until well combined, about another 5 minutes. Add the milk to the icing to make it a more spreadable consistency.
Spread the icing over the cake mixture and allow to cool in refrigerator before adding chocolate topping.
Break up the chocolate into a glass bowl and melt with the butter over a pan of simmering water. Stir well and then once melted pour and spread over the icing and again place in fridge and leave to cool. Cut into squares and enjoy!