Heat oven to 180C/160C fan/gas 4.
Grease a 20 x 30cm baking or roasting tin
and line with baking parchment. To make
the sponge batter, beat the butter, flour,
sugar, baking powder, eggs, yogurt, vanilla,
cocoa and half the coffee in a large bowl
with an electric whisk until lump-free.
Spoon into the tin, then bake for 25-30
mins until golden and risen and a skewer
poked in comes out clean. Drizzle with
some of the remaining coffee.
Cool in the tin while you stir the icing
sugar into the mascarpone. Spread over
the cooled cake, dust with a little cocoa
and scatter with the coffee beans.