To make the mascarpone ice-cream, put the mascarpone in a bowl and mix briefly to soften.
Pour in the cream, whisk gently until just combined, then fold in the condensed milk until smooth. Set aside.
To make the coffee ice-cream, put the cream, coffee powder and salt in a bowl and whip just until you have soft peaks.
Fold in the condensed milk and set aside.
Pour the brewed coffee into a wide jug and mix in the Kahlúa.
To assemble, spoon a couple of dollops each of the mascarpone and coffee ice-creams into the base of a 2 lbs loaf tin.
Dip the pieces of sponge fingers in the coffee mix, then lay a few on top of the coffee and mascarpone ice-creams.
Repeat with more scoops of the ice-creams and dipped sponge fingers, until you've used everything up.
Wrap the tin in clingfilm and freeze for at least six hours, or until firm.
When you're ready to serve, take out the ice-cream, leave it to sit and soften for five minutes, then scoop and finish with a dusting of cocoa powder.
