For the cake, mix the instant coffee and boiling water together, set aside to cool fully
Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
Make the sponge by mixing the butter (or baking spread) and golden caster sugar together in a mixing bowl until fluffy (around 3 minutes), ideally using an electric mixer
Add the eggs and whisk until fully incorporated
Add the coffee mixture, the coffee extract if using and the self raising flour. Whisk or fold them in gently
Add the chopped walnuts and fold them in
Divide the mixture between the tins, use scales for accuracy if you like
Bake them for 35-40 minutes or until a thin skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or remove from the tin and place on cooling racks
For the buttercream, mix the instant coffee and boiling water together, set aside to cool fully
Make the buttercream by mixing together the softened butter, coffee extract if using, coffee and water mixture and icing sugar until smooth. You can do this by hand, but an electric whisk or stand mixer with paddle attachment will provide the best results. If the buttercream is too stiff, you can add a little milk
Put one of the sponges on your plate or cake stand and pipe or spread some of the buttercream onto it
Add the other sponge on top and pipe or spread the remaining buttercream on top
Decorate with the walnuts, I put whole ones around the edge and chopped in the centre, but you can decorate however you would like to
Leftovers will keep in an airtight container in a cool place for 3 days