Beat all of the cake ingredients in a mixing bowl or a food processor until smooth.
Divide the mixture evenly between 2 x 18 cm (7 inch) sandwich tins, greased and base-lined with oiled greaseproof paper, and spread the surfaces level. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes until well risen, the cakes are browned and spring back when gently pressed with a fingertip.
Leave the cakes for a few minutes then loosen the edges, turn out on to a wire rack and peel off the lining paper. Leave to cool.
Make the frosting. Put the butter and half the icing sugar in a mixing bowl, add the dissolved coffee and beat until smooth. Gradually mix in the remaining icing sugar until pale and creamy.
Put one of the cakes on a serving plate, spread with half the frosting then cover with the second cake. Spread the remaining frosting over the top. Pipe or drizzle swirls of melted chocolate on top. This cake can be stored in a cake tin for 2–3 days in a cool place.