1. Preheat the oven to 180°C, gas mark 4. Butter and base-line 2x20cm sandwich tins with baking paper. Beat the butter with the sugar until pale and light. Beat in the eggs a little at a time.
2. Mix the coffee with 2 tbsp boiling water. Fold the sifted flour into the cake mixture followed by the coffee and whiskey. Spoon into the prepared tins and level the surface. Bake for 30 minutes until well-risen and firm to touch. Rest in the tins for a couple of minutes then turn onto wire racks to cool completely.
3. For the icing, beat the butter with 25g of the mascarpone then beat in the icing sugar, reserving 2 tbsp to decorate, until very pale and light. Mix the coffee granules with a teaspoon of boiling water and then beat into the icing. Gradually beat in the whiskey. Sandwich the cakes together with the buttercream.
4. Beat the remaining mascarpone with the reserved icing sugar and spread over the top of the cake. Decorate with the chocolate coffee beans.