Preheat the oven to 180C for fan-assisted or 160C for gas.
Butter two sandwich tins and line them with baking paper.
In a bowl, add the flour, sugar, eggs, baking powder, and coffee. Stir until smooth, then fold in the walnuts.
Split the mixture between the two cake tins and smooth the tops.
Bake for approximately 15 to 20 minutes.
Remove the cakes from the oven and let them cool in the tins for about five minutes.
Run a knife around the edges to lift the cakes out and remove the parchment paper.
Beat the butter for the icing and gradually add the icing sugar until smooth. Stir in the coffee.
Spread icing on the bottom cake layer, sandwich the cakes together, and spread remaining icing on top. Decorate with walnut pieces.
