Pre heat oven to 180 C . Grease and line the bases of 2 20 cm/8 inch sandwich tins.
Cream together the butter and muscovado sugar until pale and fluffy. Add eggs one at a time mixing well. Beat in the coffee.
Sift the flour and baking powder into the mix, then fold in lightly and evenly with a metal spoon. Fold in the walnut pieces.
Divide the mixture into the prepared tins and smooth the top.Bake for 20-25 minutes or until golden brown and springy to the touch. Turn out on to a wire rack to cool.
Frosting- beat together the butter and icing sugar, coffee and vanilla essence, until mix is smooth and creamy.
Use half of the frosting to sandwich the cakes together, then spread the remainder frosting on top and swirl with a palette knife.Decorate with walnut halves. Serves 8.