Preheat the oven to 340°F and grease and line three 8-inch cake pans with parchment paper.
In a standard mixer with the paddle attachment, cream together the butter and light brown sugar until fluffy. Add the eggs one at a time and mix in between.
Set aside a handful of walnuts for decoration. Then chop the rest medium fine, leaving some chunky bits for texture, and add to the butter mixture. Mix until combined.
In a separate bowl, whisk together the flour, baking powder and salt, then mix this into the butter mixture until combined. Add the brewed espresso and Fernet and mix until smooth.
Divide the mixture evenly between the three pans and smooth the tops. Bake for 35–40 minutes until springy to touch or when a toothpick comes out clean. Invert pans and tap lightly to remove cakes. Place on rack and let cool completely.
While the cakes are baking, prepare the buttercream. Add the espresso, cream, unsalted butter, and Fernet to a large bowl and mix well. Then, slowly add up to 3 cups of powdered sugar and mix for 5 minutes. If the icing is soft, add the remaining sugar.
Assemble the cake: Line one pan with clear plastic wrap and place a layer of cake inside. Spread with a quarter of the icing. Place the next layer on top and repeat. Top with the last layer of sponge. Cover the cake with plastic wrap and chill in the fridge for at least 30 minutes. Remove from fridge, unwrap and turn out onto a plate. Frost cake as desired. For garnish, grate walnuts with a zester or grater over the top of the cake. Serves 10-12.
