A pretty white-iced triple-layer cake to make in autumn, when walnut are harvested, this looks glamorous enough for an anniversary or other important occasion.
Preheat the oven to 160°C/140°C/Gas 3. Grease three 20cm sandwich tins and line the base of each tin with baking parchment. Mix the flour, baking powder and walnuts together in a bowl.
In a separate bowl, cream the butter and sugar together until pale and fluffy. Beat in the eggs a little at a time.
Fold in the flour mixture using a large metal spoon. Divide the mixture equally between the three tins and level the surfaces.
Bake in the pre-heated oven for about 25–30 minutes until the cakes are golden and springy to the touch. Leave to cool in the tins for a few minutes then turn out, peel off the parchment and finish cooling on a wire rack.
For the caramelised walnuts, tip the sugar into a pan and heat over a low heat until the sugar is beginning to melt. Increase the heat and cook the caramel to a dark golden colour. Remove from the heat, add the walnut halves and swirl to coat in the caramel. Transfer the walnut halves onto a silicon sheet and leave to set.
For the buttercream, put the butter, vanilla extract, 1 tablespoon of the milk and half the icing sugar in a large bowl and beat until smooth. Beat in the remaining icing sugar and pour in the rest of the milk, if needed, to make the buttercream the right consistency.
Spread half of the buttercream over one of the sponges and place the second sponge on top. Spread the remaining buttercream over the second sponge and top with the third sponge.
For the boiled icing, measure the egg whites, sugar, water and cream of tartar into a bowl set over a pan of hot water and whisk for 8–10 minutes until thick.
Cover the top and sides of the cake, swirling the icing to form softened peaks. Work quickly as the icing sets rapidly. Leave to set in a cool place, but not in the fridge. Decorate with the caramelised walnuts.