- Preheat the oven at 160°C. Baseline and grease two equally sized sandwich tins. Add the sugar and the butter to a bowl and whisk until very fluffy and a pale cream.
- Whisk the eggs in a mug with a fork and then add them gradually to the mixture with 1 tbsp of flour each time. Make sure you don’t use all the flour.
- Add the rest of the flour and the baking powder to the mixture and fold it in gently.
- Dissolve the coffee in the boiling water and add to the mixture still folding. Divide into the cake tins and bake for 30 minutes
- Meanwhile cream the butter and the icing sugar until light and fluffy. Dissolve the coffee in boiling water, making sure you don’t add too much water or the icing will be runny and add it to the butter and icing sugar. Whisk and leave in the fridge until the cake is done.
- Once the cakes are done and have been put onto plates spread the icing on the bottom of one of the cakes (leaving around half of the icing for the top). Spread the remaining icing on top of the cake. Decorate with walnuts.