Photograph: Jonathan Gregson
For the ice cream, place the spices in a saucepan, bash with the end of a rolling pin and toast over a medium heat for 2 minutes, or until fragrant. Add the milk and bring to the boil, then remove from the heat and set aside for 10 minutes.
Whisk the egg yolks with 50g of the sugar, then whisk in a little of the hot spiced milk. Return the mix to the pan of spiced milk and stir over a low heat until the custard is thick enough to coat the back of a spoon. Be careful not to let it boil or it may split. Add 100ml of the cream to the custard, then pour through a fine sieve into a shallow container and discard the spices. Cover the surface of the custard with clingfilm; cool.
Whip the remaining 200ml of cream to soft peaks. In a separate bowl, whisk the egg whites until soft peaks form, then whisk in the remaining 100g of sugar until smooth and glossy. Fold the cold custard and whipped cream together, then gently fold in the egg whites in batches. Transfer the mixture to a suitable container for freezing and cover with a lid; freeze.
For the mousse, soak the gelatine in a small bowl of cold water for 10 minutes. Put the chocolate in a bowl and heat the vanilla custard in a pan to just below simmering point. Squeeze out the excess liquid from the gelatine and stir into the hot custard until dissolved. Tip the hot custard on to the chocolate and stir to melt; cool. In another bowl, mix the Armagnac into the cream and whisk until the cream forms soft peaks, then fold into the cooled chocolate mixture. Pour into an 8.5cm-deep 17 x 10cm loaf tin lined with clingfilm; cover and chill.
For the prunes, gently heat the port and Armagnac in a small pan. Add the prunes and simmer very gently until most of the liquid has been absorbed. Whiz to a purée and set aside. For the crumble, preheat the oven to 200°C, fan 180°C, gas 6. Thinly slice the malt loaf and toast on a baking tray in the oven for 7-8 minutes, turning halfway through, until crisp, then whiz to fine crumbs in a food processor. Return to the oven briefly to crisp, if needed.
To serve, turn the mousse out and remove the clingfilm. Scatter with most of the crumble and slice. Add some prune purée to each plate and a slice of mousse. Top with a scoop of ice cream and a sprinkling of crumble.
Get ahead: make the mousse, prune purée and crumble up to 3 days ahead. Make the ice cream up to a month ahead, scooping it into balls and re-freezing to make life easy when you come to serve the dessert.