For the vanilla ice cream, place the milk, cream, vanilla pod and seeds into a heavy-based pan and bring to the boil. Remove from the heat, then discard the vanilla pod.
Whisk the egg yolks and sugar together in a bowl until pale and fluffy, then slowly whisk in the milk and cream mixture until well combined. Return the custard to a clean pan and cook over a medium-low heat for 5-6 minutes, until it thickens enough to coat the back of a wooden spoon. Set aside to cool completely.
Pour the custard mixture into an ice cream machine and churn according to the manufacturer's instructions. Spoon into a freezer-proof plastic container and chill in the freezer until ready to serve. Remove from the freezer five minutes before serving to soften.
For the chocolate crumble, preheat the oven to 210C/410F/Gas 6.
Sift the flour, sugar and cocoa powder into a bowl. Add the butter and rub together with your fingertips until the mixture resembles breadcrumbs. Spread the mixture out on a baking tray and bake in the oven for 10-12 minutes, or until golden-brown and crisp. Shake the tray halfway through the cooking time to prevent the crumbs from sticking together too much. Remove from the oven and set aside to cool.
For the chocolate foam, place the chocolate and cream into a heatproof bowl set over a pan of just-simmering water (make sure the bottom of the bowl doesn't touch the water). Leave until melted, then whisk in the egg whites until well combined.
Spoon the mixture into the foam gun and gently warm through in the pan of simmering water. Spray the chocolate foam into four serving bowls.
To serve, top the chocolate foam with the crumble mixture and serve with a scoop of vanilla ice cream.