Pre-heat oven to 180 C. Prepare a 22cm tin by lightly buttering it, then shake a thin film of flour around the tin and discard any excess. Combine the coffee, sugar and butter in a small bowl and heat in microwave for 2 minutes until the butter has melted. Add chopped chocolate and stir until smooth and blended. Remove from heat and add ground nuts. Mix well then beat in egg yolks one at a time. In another bowl beat egg whites until stiff. Stir in a couple of spoonfuls into the chocolate mix to lighten the mixture then fold into the rest of the mix. Turn into the cake tin. Bake for 40-50 minutes until a skewer comes out clean from the centre of the cake. Leave to cool in the tin and don’t worry if the crust collapses – this is normal!