Preheat oven to 180C/350F/Gas Mark4. Grease and line the bases of two 20cmx8inch sandwich tins.
Cream the butter and sugar until pale and fluffy. Can mix with electric hand mixer or a stand mixer. Gradually add the eggs, beating well after each addition. Beat in the coffee.
Sift the flour and baking powder into the mixture, then fold in lightly and evenly with a metal spoon. Fold in the walnuts.
Divide the mixture between the prepared tins and smooth the top. Bake in preheated oven for 20-25 minutes or until golden brown and springy to the touch. Turn out onto a wire rack to cool.
Frosting
Beat together the butter and icing sugar, coffee and vanilla mixing until smooth and creamy.
Use about half of the mixture to sandwich the cakes together, then spread the remaining frosting on top with a palette knife. Decorate with walnut halves.