Preheat oven to 180 degrees C. Lightly grease and flour three 23cm round cake tins.
Place the sugar and butter in a mixing bowl and 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. Stir the remaining buttermilk into the egg mixture, beating until smooth.
Stir the bicarb soda into 1 tablespoon buttermilk until smooth then 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 toothpick or small knife inserted into the centre comes out clean, about 30 minutes. Remove from the oven and cool in the pans about 15 minutes before turning out onto a wire rack to cool completely.
To make the icing, beat 250g butter until smooth and creamy. Gradually sift 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 icing. Place the second layer on top of the first, and ice. Repeat with the third layer. Use remaining icing to ice top and sides.