Preheat the oven to 180℃ fan/200℃/Gas Mark 6.
Grease 2 x 7" round sandwich tins with butter and line the bases with baking paper or baking parchment.
Cream your butter and sugar until it’s pale, light and fluffy. Use an electric hand mixer if you have one.
Mix in the eggs, one at a time, beating really well after each egg. Don’t worry if it starts to look a bit curdled, it’ll all come together.
Mix in your flour. If you’ve been using a mixer, do this bit by hand. It helps to keep the sponge light.
Stir in the coffee and water mixture along with the chopped walnuts.
Spoon into your tins and smooth out with the back of a spoon. Make a bit of a dip in the middle of each sponge to account for rising and keep them level.
Bake for 25 - 35 minutes or until a skewer comes out clean. Allow the cakes to cool in the tins for 5 minutes. Remove from their tins and allow to cool completely.
Meanwhile, make your icing. Beat your butter until it's pale, light and fluffy.
Beat in your icing sugar. Stir in your coffee mixture. (It should be light and fluffy.)
When your cakes are completely cold, sandwich each layer together with your buttercream, and a layer on top. Pipe some decoration on top if you have a piping bag, before decorating with walnut halves.
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