Preheat the oven to 180°C/160°C Fan/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/2 sticks butter, flour, 4 teaspoons espresso powder, 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.

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.

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 and cover the top with the remaining icing in a ramshackle swirly pattern.

Gently press the walnut halves into the top of the icing all around the edge of the circle about 1cm apart.