Heat oven to 180C/fan 160C/gas 4. Butter 2 x 20cm sandwich tins and line the bottoms with greaseproof paper. Beat butter and sugar together with electric beaters until pale and creamy. Add the fl our and eggs in one go and keep beating until evenly mixed. Fold in the walnuts (if using) and half of the coffee. Spoon the mix into the prepared tins and bake for 25-30 mins or until golden and well risen.
Leave the cakes in their tins for 5 mins before turning onto a wire rack. Sweeten the remaining coffee with the extra sugar and sprinkle 4 tbsp over the sponges. Leave to cool completely.
While the cakes cool, make the frosting. Tip the mascarpone into a large bowl and beat in the sugar and remaining coffee until smooth and creamy. Use about half of the frosting to sandwich the sponges together then, using a palette or cutlery knife, spread the rest of the frosting over the top of the cake. Decorate with a dusting of cocoa powder or drinking chocolate. If you’re making this cake to eat at home, it will keep covered in the fridge for 2-3 days.