Preheat oven to 180 C / Gas 4. Lightly grease and flour three 23cm round cake tins.
Place the sugar and butter in a mixing bowl. Beat until light coloured and smooth. Beat in the eggs one at a time. Stir the coffee and cocoa into the egg mixture until well blended. Measure the buttermilk and remove 1 tablespoon, placing it in a separate small bowl and set aside. Stir the remaining buttermilk into the egg mixture, beating until smooth.
Stir the bicarbonate of soda into 1 tablespoon buttermilk until smooth and mix into the egg mixture. Stir in the flour, beating until smooth. Pour the batter evenly into the prepared cake tins.
Bake in preheated oven until a skewer or small knife inserted into the centre comes out clean, about 30 minutes. Cool in the tins for about 15 minutes before turning out onto a wire rack to cool completely.
Meanwhile, beat 225g butter until smooth and creamy. Gradually sieve in the icing sugar and beat until smooth. Stir in the coffee, cocoa and vanilla until well blended.
Place one cake layer on a serving plate. Spread top with about 1/5 of the icing. Place the second layer on top of the first and ice. Repeat with the third layer. Use remaining icing to spread over the top and sides.