Steps
- Preheat the oven to 350°F / 180°C. Butter two 8in / 20cm round cake pans and line their bases with parchment paper.
- Process 50g of walnut pieces with 225g of superfine sugar to a fine powder.
- Add 225g of softened unsalted butter, 200g of all-purpose flour, 4 teaspoons of instant espresso powder, 2½ teaspoons of baking powder, ½ teaspoon of baking soda, and 4 large eggs. Process until smooth.
- Pulse in 1 tablespoon of milk, adding a little more if needed for a soft batter that drops from a spoon.
- Divide between the pans and bake for 25 minutes, until risen and springy. Cool in the pans for 10 minutes, then turn onto a rack, remove the paper, and cool completely.
- For the frosting, process 350g of icing sugar until lump-free, then blend in 175g of softened unsalted butter.
- Dissolve 2½ teaspoons of instant espresso powder in 1 tablespoon of boiling water. Pulse the hot coffee into the frosting until smooth.
- Place one cake upside down on a platter and spread with half the frosting. Add the other cake, right side up, swirl the remaining frosting over the top, and decorate with walnut halves.