250g pack softened butter, plus extra for the tins 100ml strong black coffee (made with 2 tbsp coffee granules), cooled 280g self-raising flour 250g golden caster sugar ½ tsp baking powder 4 egg 1 tsp vanilla extract 85g walnut, 2 tbsp roughly chopped, the rest finely chopped 100g icing sugar, sifted, plus a little extra for dusting 150ml double cream 100g mascarpone, at room temperature