1.Preheat oven to 180°C/160°C fan-forced. Line two 12-hole (⅓-cup/80ml) muffin pans with paper cases.
2.Beat butter in large bowl with electric mixer until pale. Beat sifted flour, cocoa and ¼ cup of the caster sugar alternately with milk into butter, in two batches, until combined.
3.Beat eggs and egg whites in small bowl with electric mixer until thick and creamy. Gradually add remaining caster sugar, one tablespoon at a time, beating until sugar dissolves between additions. Gradually beat egg mixture into flour mixture until combined.
4.Spoon 2½ tablespoons mixture into each paper case; bake about 20 minutes. Turn, top-side-up, onto wire rack to cool.
5.Meanwhile, make chocolate icing. Spread cold cakes with icing.chocolate icing Sift sugar and cocoa into small bowl; stir in water until smooth.