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Put the double cream, mascarpone, marsala and golden caster sugar in a large bowl.
Whisk until the cream and mascarpone have completely combined and have the consistency of thickly whipped cream.
Pour the coffee into a shallow dish. Dip in a few of the sponge fingers at a time, turning for a few seconds until they are nicely soaked, but not soggy. Layer these in a dish until you’ve used half the sponge fingers, then spread over half of the creamy mixture.
Using the coarse side of the grater, grate over most of the dark chocolate. Then repeat the layers (you should use up all the coffee), finishing with the creamy layer.
Cover and chill for a few hours or overnight. Will keep in the fridge for up to two days.
To serve, dust with the cocoa powder and grate over the remainder of the chocolate.