Heat oven to 190C/fan 170C/gas 5. Butter two 20cm sandwich tins and line with non-stick baking paper. Toast the walnut halves in the oven for 10 mins. Reserve 10 and chop the rest. Dissolve the instant coffee in 1 tbsp of boiling water, cool and add the basic cake mix (see recipe for Classic Victoria sponge, right), instead of the milk.
For the filling, beat the mascarpone with the light muscovado sugar and Tia Maria until smooth. Sandwich the cooled cakes with a third of the filling and the chopped walnuts.
Cover the top and sides of the cake with the rest of the filling and decorate the top with the toasted walnut halves.