Preheat the oven to 180°C, (160°C for fan ovens) Gas Mark 4. Grease and base line the cake tin.
Place the sponge ingredients except the walnuts into a large mixing bowl and beat until light and creamy with an electric hand mixer.
Fold in the chopped walnuts and spoon the mixture into the tin. Bake for about 45-50 minutes, or until well risen and a skewer comes out clean.
When the cake is cold, beat the ingredients for the filling together in a bowl until you have a soft buttercream. Split the cake into two or three layers and then sandwich back together with the filling.
Gradually mix the coffee liquid into the fondant icing sugar, adding more water, in drops, to form a thick icing.
Spread the icing onto the centre of the cake and coax it to the sides. Sprinkle with walnuts if using and leave to set.