Preheat the oven to 150 degrees C and grease a 20x30cm cake tin.
In a small bowl, mix together 1/2 cup cocoa and 3/4 cup boiling water; set aside.
In another small bowl, mix together the sour cream and bicarb soda.
In a large bowl, cream the 125g butter and 2 cups sugar. To the butter and sugar mixture, alternately add the sifted flour and the cocoa mixture with the sour cream mixture. Beat until fluffy. Beat the egg whites until stiff and fold in the egg whites and 1 1/2 teaspoons of vanilla.
Pour batter in the prepared tin and bake 50 minutes.
To make icing:
Cream 90g butter in a small bowl. Add 3/4 cup cocoa and icing sugar alternately with milk; beat until spreading consistency. More or less milk can be used depending on the texture you want. Stir in the vanilla and ice the cake.