100g butter, well softened
100g light muscovado sugar
100g self-raising flour
2 large egg
2 tsp instant coffee, mixed with 100ml/3½fl oz boiling water, then cooled
25g walnut half, chopped, plus 12 more for the tops
For the topping
200ml tub mascarpone (you'll find this with the soft cheeses)
2 tbsp light muscovado sugar