Whisk together the cooled icing ingredients. Cover and chill for 3-4 hours.
Preheat the oven to 180°C/Gas 4/fan oven 160°C. Butter & flour two 20cm/8" loose-based cake tins. Sift the flour and baking powder. Stir the vanilla into the egg yolks. Heat half the milk to boiling point, stir in the coffee to dissolve, then add the rest of the milk and cool.
Cream the butter and 200g/8oz of the caster sugar. Slowly whisk in the egg yolk mixture, then the melted chocolate. Fold in the sifted dry ingredients, the cooled milk and the soured cream. Whisk the egg whites until stiff; whisk in the remaining sugar to form firm peaks. Fold the egg whites into the cake mixture and pour into the tins.
Bake for 30 minutes until risen. Cool, split each cake in two, and layer with the icing.
The cake is delicious when warmed slightly - 30 seconds in a microwave will do it. Serve with crème fraîche, double cream or a scoop of vanilla ice cream. We added some mini chocolate chips for decoration - chocolate covered coffee beans would also work well!