Preheat oven to 160°C. Grease a deep 22cm square cake tin with some melted butter then line base and sides with baking paper.
Sift the flours, bicarbonate of soda and cocoa. Add the caster sugar and make a well in the dry ingredients.
Melt 250g dark chocolate and 250g butter over low heat. Add 3/4 cup water and stir until combined. Gradually stir in the chocolate mixture into the dry ingredients.
Whisk the buttermilk, eggs, coffee and oil, then add to the chocolate mixture and stir until smooth. Pour the mixture into the prepared cake tin and bake for 11/2 hrs or until skewer comes out clean when inserted into the middle. Remove from the oven and allow to cool in the tin for 10 mins before turning out onto a rack.
Chocolate icing. Melt remaining dark chocolate and butter over low heat and stir until smooth. Cool to near room temperature, stir often until it is thickened enough to spread over cooled cake. Allow icing to set before serving.