Heat oven to 180C/160C fan/gas 4.
Butter 2 x 20cm round cake tins and
line with baking parchment. Set aside
1 tbsp of the coffee for the filling.
Beat the butter, flour, sugar, baking
powder, eggs, vanilla and half the
remaining coffee in a large bowl with
an electric whisk until lump-free. Fold
in the finely chopped walnuts, then divide
between the tins and roughly spread.
Scatter the roughly chopped walnuts
over one of the cakes. Bake the cakes
for 25-30 mins until golden and risen,
and a skewer poked in comes out clean.
Drizzle the plain cake with the remaining
coffee. Cool the cakes in the tins.
Meanwhile, make the filling: beat
together the icing sugar, cream and
mascarpone, then fold in the reserved
2 tbsp coffee. Spread over the plain cake,
then cover with the walnut-topped cake
and dust with a little icing sugar.