This recipe is by Richard Burr from The Great British Bake Off. I modified the recipe to just one cake instead of 18 mini-cakes.
For the sponge:
unsalted butter, for greasing
75g/2½oz walnut pieces
225g/8oz caster sugar
225g/8oz unsalted butter
175g/6oz plain flour
3 tsp instant decaffeinated coffee granules, dissolved in 1 tbsp boiling water
2½ tsp baking powder
½ tsp bicarbonate of soda
4 large free-range eggs
4 tsp whole milk
For the filling and topping:
500g/1lb 2oz icing sugar
175g/6oz unsalted butter
2½ tsp instant decaffeinated coffee, dissolved in enough boiling water to make a smooth paste
3-4 tsp lustre dust (optional)
100g/3½oz walnut pieces, crushed
chocolate coffee beans, to decorate (optional)
For the sponge, preheat the oven to 325F and grease and line two 9x13in baking trays with baking parchment.
For the sponge, put the walnuts and sugar in a food processor and grind to a coarse powder consistency. Transfer to a large bowl or a freestanding mixer. Add butter to the bowl and cream until smooth and light. Sift in the flour and continue to mix. Add all the remaining ingredients and mix to a smooth consistency.
Divide the mixture equally between the prepared tins and smooth the surface. Bake for 10-12 minutes, or until golden-brown and springy to touch. Leave to cool for a few minutes before turning out of the tins onto wire racks.
When the cakes are completely cool, split the cakes into two layers.
For the filling, sift 350g/12oz icing sugar into a large bowl or a freestanding mixer. Add the butter, half the coffee mixture and the lustre dust and mix until you have a smooth buttercream icing with a soft golden shimmer. Transfer to a piping bag fitted with a plain nozzle.
For the topping, mix the remaining coffee paste with four tablespoons of water. Sift the remaining icing sugar into a bowl and stir in the coffee mixture to make glacé icing for the tops of the cakes. If the icing is very thick add more water until the icing runs off a spoon in thick ribbons.
To assemble the cakes, pipe coffee buttercream icing on top of one. Top with another layer of cake and pipe more buttercream on top. Top with another layer of cake and pipe more buttercream on top. Add a final layer of cake and drizzle the glacé icing over. Top with crushed walnut pieces and chocolate coffee beans, if using.