Line a round 20-23cm (8-9in) loose-based springform cake tin with non-stick baking parchment. Dissolve the coffee in the boiling water and mix with the brandy.
Break the eggs into a bowl, add the sugar and whisk together at high speed until thick and frothy – the mixture should leave its own trail when trickled from a height. Blend a little of this mixture with the mascarpone in another bowl, then stir in the remaining egg and sugar mixture.
Whisk the cream until thick and fold into the egg and mascarpone mixture. Coarsely chop the chocolate chips in a food processor to give chocolate bits and powder.
Split the sponge fingers and line the prepared tin with half of them. Sprinkle over half the coffee mixture and scatter over one-third of the chocolate, then add half the mascarpone mixture. Add a second layer of sponge, then the remaining coffee mixture and another third of the chocolate.
Cover with the rest of the mascarpone mixture and sprinkle with the remaining chocolate. Chill for four hours, then turn out of the tin, remove the parchment and serve very cold.
