Grease a 7cm-deep, 20cm (base size) square cake pan and line the base and sides with baking paper, allowing the paper to overhang the sides.

Place the biscuits in a food processor and process until coarsely crushed. Add the butter and process until well combined. Use the back of a spoon to press the mixture evenly over the base of the prepared pan. Place in the fridge for 15 minutes to chill.

Meanwhile, use electric beaters to beat the egg yolks and sugar in a large bowl until pale and creamy. Add the mascarpone and cream. Beat until soft peaks form. Add 125ml (½ cup) coffee and beat until well combined. Pour the remaining 250ml (1 cup) coffee into a shallow bowl and set aside.

Place the water in a small microwave-safe bowl. Sprinkle with the gelatine and stir until well combined. Microwave for 10 seconds (do not overheat). Use a fork to whisk until the gelatine dissolves. With the beaters running, add the gelatine mixture to the mascarpone mixture. Beat until well combined. Spread ⅓ cup of the mascarpone mixture over the biscuit base. One at a time, quickly dip the sponge finger biscuits into the reserved coffee and arrange in a single layer in rows over the mascarpone layer, trimming to fit if necessary. Spoon over the remaining mascarpone mixture. Smooth the surface. Place in the fridge for 6 hours or overnight to set.

Dust the slice with cocoa and slice to serve.