Preheat the oven to 180°C, (160°C for fan ovens) Gas Mark 4. Grease and base line the cake tin.
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Place the sponge ingredients except the walnuts into a large mixing bowl and beat until light and creamy with an electric hand mixer.
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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.
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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.
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Gradually mix the coffee liquid into the fondant icing sugar, adding more water, in drops, to form a thick icing.
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Spread the icing onto the centre of the cake and coax it to the sides. Sprinkle with walnuts if using and leave to set.