for the sponge
50g walnut pieces
225g caster sugar
225g soft unsalted butter (plus some for greasing)
200g plain flour
4 teaspoons instant espresso powder or make a strong coffee, which is what I did.
2 ½ teaspoons baking powder
½ teaspoon bicarbonate of soda
4 large eggs
1 – 2 tablespoons milk
for the buttercream frosting
350g icing sugar
175g soft unsalted butter
2 ½ teaspoons instant espresso powder (dissolved in 1 tablespoon boiling water, or see above)
approx. 10 walnut halves (to decorate)
Preheat the oven to 180°C/gas mark 4/350°F.
Butter two 20cm / 8inch sandwich tins and line the base of each with baking parchment. Put the walnut pieces and sugar into a food processor and blitz to a fine nutty powder. Add the 225g butter, flour, 4 teaspoons espresso powder/coffee, baking powder, bicarb and eggs and process to a smooth batter.
Add the milk, pouring it down the funnel with the motor still running, or just pulsing, to loosen the cake mixture: it should be a soft, dropping consistency, so add more milk if you need to. (If you are making this by hand, bash the nuts to a rubbly powder with a rolling pin and mix with the dry ingredients; then cream the butter and sugar together, and beat in the dry ingredients and eggs alternately and, finally, the milk.)
Divide the mixture between the 2 lined tins and bake in the oven for 25 minutes, or until the sponge has risen and feels springy to the touch.
Cool the cakes in their tins on a wire rack for about 10 minutes, before turning them out onto the rack and peeling off the baking parchment. When the sponges are cool, you can make the buttercream. Pulse the icing sugar in the food processor until it is lump free, then add the butter and process to make a smooth icing. Dissolve the instant espresso powder in 1 tablespoon boiling water and add it while still hot to the processor, pulsing to blend into the buttercream.
If you are doing this by hand, sieve the icing sugar and beat it into the butter with a wooden spoon. Then beat in the hot coffee liquid.
Place 1 sponge upside down on your cake stand or serving plate.
Spread with about half the icing; then place on it the second sponge, right side up (i.e. so the 2 flat sides of the sponges meet in the middle) and cover the top with the remaining icing in a ramshackle swirly pattern.
This cake is all about old-fashioned, rustic charm, so don’t worry unduly: however the frosting goes on is fine. similarly, don’t fret about some buttercream oozing out around the middle: that’s what makes it look so inviting.
Gently press the walnut halves into the top of the icing all around the edge of the circle about 1cm apart