Method

1. To make your mousse, place the chocolate in a large bowl set over a pan of barely simmering water (making sure the bottom of the bowl does not touch the surface of the water) and leave to melt over a low heat. Once the chocolate is completely melted remove bowl from the pan.
2. In a separate bowl, whisk the egg whites and fructose together until the soft peak stage.
3. Whisk a third of the egg whites into the melted chocolate mixture, then fold in the remaining egg white with a large spatula, taking care to maintain volume. Place in a refrigerator for two hours, whilst you prepare the crumble topping.
4. To make your crumble topping place all the ingredients in a food processor and pulse until they form pieces the size of breadcrumbs. Tip out onto a board and bring the dough together by hand. Flatten the dough into a thin disk, wrap in cling film and freeze for half an hour. Preheat your oven to 170°c. Remove the dough from the freezer, break into several pieces and pulse again in the food processor until mixture is just broken into small chunks. Sprinkle mixture evenly onto a baking tray lined with greaseproof paper. Place in preheated oven and bake for 10-12 minutes. Allow to cool, then lightly break up any clumps with your fingertips.
5. To serve, place a generous scoop of the ice cream into the bottom of a sundae glass. Spoon a layer of mousse mixture over the ice cream and sprinkle the chocolate crumble over the top. To finish, dust with icing sugar.