In defense of the walnut, its nubbly and oily texture is far superior to the sharper, drier hazelnut. No matter, they just need a but more work before you add them to the sponge batter. So toast them to make the flavour really stand out, and rub off the unpleasant scratchy skins as thoroughly as you can. They need to be chopped quite finely, to avoid sudden, unpleasant hard lumps as you eat. But that's it, and its worth the small extra effort.
The recipe is essentially a Victoria sponge base, with some instant coffee, nuts and hazelnut liqueur. I looked up Nigella Lawson's recipe, and she uses espresso for a more intense coffee flavour. Not being a coffee drinker (just a coffee cake eater), I prefer the sweeter, more muted version, but you can replace the instant coffee with 4 teaspoons espresso if you like things a bit stronger. Also her icing uses 3 tsp espresso coffee in place of the instant I used.
For the sponge
225g soft unsalted butter (plus some for greasing)225g caster sugar
200g plain flour
2 tsp coffee granules, dissolved in 1tbsp boiling water
3 tsp baking powder
1 tsp bicarbonate of soda
4 medium eggs, beaten
2 tbsp Frangelico (hazelnut liqueur) or milk
For the buttercream frosting350g icing sugar
175g unsalted butter, very soft
2 tsp coffee granules dissolved in 1 tbsp boiling water
3 tbsp Frangelico
25g hazelnuts, from the 75g above
1-2 tsp cocoa
Preheat the oven to 200C. Line two 20cm sandwich cake tins with parchment, and butter the sides.
In a dry (non-oiled) frying pan, toast the hazelnuts over a medium heat, until their skins darken and they start to release their scent. Transfer them into a bowl- if you leave them in the hot pan they will continue to cook and probably burn. When they are cool enough to handle, rub them with your fingers to remove as much of the skin from them as possible. Chop 50g of them finely, and reserve the remaining 25g for decoration.
Cream the butter until it is very soft and light, and gradually beat in the sugar. Beat in a quarter of the beaten egg along with a spoonful of flour. Continue this until all the egg is used up. Now sift in the rest of the flour along with the baking powder and the bicarb. Fold this in, and then add coffee, nuts and Frangelico. Divide between the two cake tins, and bake for about 25 minutes.
Make sure the cakes are cool before you start decorating. For the icing, beat the butter to make sure it is truly soft. Sift in the icing sugar, beating it into the butter in stages. Once all the icing sugar has been incorporated, stir in the hot coffee liquid and frangelico.
Put one of the sponges face down on a plate. Spread the top surface with about a third of the icing. Place the other sponge, facing the right way up, on top. Spread with the remaining frosting, and decorate with the remaining hazelnuts. Sift the cocoa powder on top.