Start by preheating your oven to 180C/160C fan/gas 4 and grease and line three 8” cake tins.
In the bowl of a stand mixer (or a large mixing bowl), cream together the butter and sugar until light, fluffy and pale in colour.
Crack the eggs into a separate bowl and add the coffee extract and the instant coffee powder. Beat until smooth.
Running the mixer on a low speed, begin to add the beaten eggs to the butter/sugar mixture a little at a time. If the mixture starts to curdle, add a tablespoon of flour.
Sieve half of the flour into the bowl and mix until just combined, before repeating with the remaining flour. Be careful not to over-beat the mixture.
Finally, fold through the walnuts with a spatula or a large metal spoon.
Divide the mixture equally between the three tins and level off with your spatula, before baking in the preheated oven for around 30 minutes (until the cakes are soft and springy to the touch and a skewer comes out clean).
Once your cakes are baked, remove them from the oven and leave to cool in the tins for around 5-10 minutes, before inverting onto a wire rack to cool down completely.
To make the coffee syrup, add the sugar, water and instant coffee powder to a saucepan and stir to combine. Place over a medium heat and allow to come to the boil, stirring regularly until all of the sugar has dissolved. Allow to boil for a further three minutes, before removing from the heat and allowing to cool to room temperature. Your coffee syrup can be placed into a syrup bottle or other airtight container and kept in the fridge for up to three months.
Finally, make the buttercream. Begin by creaming your butter in the bowl of a stand mixer until pale and smooth. This may take a while, but the longer you leave it, the better.
Add in half of the icing sugar and mix on a low speed until it is fully incorporated. Add the second half of the icing sugar and the coffee extract, and again mix until fully incorporated. This shouldn’t take more than 1 minute.
Cover the bowl of buttercream with clingfilm and set aside until ready to use.
When your cakes have fully cooled, use a cocktail stick to pierce holes all over the bottom of your cake layers. With your syrup bottle or a large spoon, completely soak the cakes with a generous amount of coffee simple syrup – this will ensure your cakes pack a real coffee punch – and set aside to dry.
Finally, when you are ready to assemble your cake, begin by giving your buttercream a final pulse in the stand mixer before transferring into a large piping bag fitted with a large round piping tip (Wilton 1A).
Place one of the cake layers right-side up onto your serving plate and pipe ‘kisses’ around the edge of the cake (see image). Fill the centre of the cake with more of your buttercream, using an offset spatula to ensure it is evenly distributed.
Add the next layer of cake and repeat the above step, piping kisses and filling the centre.
Next, add the top layer of cake and cover it entirely with piped ‘kisses’.
Finally, sprinkle over a handful of roughly chopped walnuts and set aside until ready to serve.