Mix together the coffee and amaretto in a shallow dish. Dip half the sponge fingers, one at a time into the mixture for 10 -15 seconds, turning over once. Halve the biscuits and place in the base of four glass dishes.
Place the mascarpone cheese, sugar and custard in the bowl, fit the whisk and whisk at speed 2 for 1 minute, scraping if necessary, until well combined.
Spoon half the mascarpone mixture on top of the biscuits. Dredge heavily with cocoa powder. Soak the remaining biscuits in the coffee mixture and arrange on top of the mascarpone mixture.
Spoon the remaining mascarpone mixture on top of the sponge fingers and create a swirled effect. Dredge heavily with cocoa powder and chill overnight.