Preheat the oven to 180°c/160°c fan/Gas 4. Grease and base line two 20cm round sandwich tins.
To make the sponges add all the ingredients into the mixing bowl and mix using a food mixer, until smooth and combined. Divide the batter between the tins and level the surface with the back of a spoon.
Bake in the oven for 25 minutes until golden. Set aside to cool completely before turning out the tins and removing the baking paper.
Buttercream
Measure the coffee granules into a bowl, add the boiling water and stir until smooth, then allow to cool. Add the softened butter and sifted icing sugar to the bowl and beat with an electric hand whisk until pale and fluffy.
To assemble the cake, spread half the buttercream on one sponge, top it with the other sponge and spread the remaining buttercream on top. Arrange the walnut halves on the buttercream slice and serve.
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