Preheat the oven to 180ºC/gas 4. Grease and line the bases of 2 round 20cm cake tins.
In a food processor, blitz two-thirds of the nuts to powder. Cube, then cream the butter in a bowl with the sugar until light and fluffy. Gradually beat in the eggs then add the blitzed walnuts and gently stir through.
Fold the flour, baking powder and a pinch of sea salt through with a large metal spoon until just incorporated.
Roughly chop the rest of the walnuts and stir through the cake mixture along with the espresso.
Evenly divide the mixture between the cake tins and bake in the oven for 20 to 25 minutes, or until lightly golden and cooked through when tested with a skewer.
Cool the cakes in the tins for 5 minutes, then transfer to a wire rack to cool completely.
For the filling, cube and put the butter in a bowl. Sift in the icing sugar, beat until fluffy, then beat in the coffee.
For the icing, sift the icing sugar into a bowl and beat in the coffee to make a thick, smooth icing.
Place one of the cakes on a plate or board and spread with the coffee filling. Top with the second cake and drizzle over the icing.
Decorate the cake with the remaining walnuts as they are, or lightly caramelised. To do this, melt sugar over a medium heat, without stirring, then carefully toss the nuts in briefly to coat. Cool the nuts on a wire rack before using.