Preheat the oven to 160°C/325°F/Gas mark 3.
Lightly grease a deep 23cm/9in
round
cake tin and line with baking parchment, making
sure the paper around
the side extends at least 5cm/2in above the top
edge.
Put the butter, chocolate and coffee in a pan
with the hot water and stir over a
low heat until smooth. Remove from the heat.
Sift the flours, cocoa and bicarbonate of soda
into a large bowl. Stir in the sugar and make a
well in the centre.
Combine the eggs, oil and
buttermilk, add the dry ingredients and, using a
large metal spoon, slowly stir them in.
Gradually stir in the butter mixture.
Pour the mixture into the tin and bake for 1 1/2
hours. Remove from the oven. If
the top looks raw, bake for another 5 to 10
minutes, then remove. Leave in the tin until
completely cold, then turn out.
For the icing, combine the butter and chocolate
in a saucepan and stir over a low heat until
melted.
Remove and allow to cool slightly.
Spread over the cake and serve.