1Put a 1½ litre pudding basin into the freezer to get cold. For the blood orange ice cream, put the egg yolks, the whole egg and the sugar into a bowl and whisk until pale, thick and creamy. Pour the cream and milk into a pan, add the zest and bring almost to the boil. Just before it boils, turn off the heat and leave to cool for 10 minutes before stirring into the egg and sugar mixture.
2Rinse and dry the pan, then pour the custard mixture back into it. Place over a lowish heat and stir regularly with a wooden spoon for about 10 minutes or until the mixture is starting to thicken a little. Remove from the heat, continue stirring, and cool quickly by placing the pan in a sink of cold water.
3When the custard is cool, stir in the blood orange juice and extract, and churn in an ice-cream machine. If you chill the mixture for an hour before you churn it, it will speed up the churning process. If you don't have an ice-cream machine, tip the custard into a freezerproof bowl and freeze for 1 hour. As it starts to freeze around the edges, stir vigorously with a spatula, or whisk it, to break it up, then return it to the freezer. Check every 30 minutes, stirring vigorously, until the ice cream is frozen; this will take 3-4 hours.
4Remove both the orange and vanilla ice creams from the freezer to allow them to soften slightly.
5Remove the basin from the freezer. Using a large palette knife and working quickly, spread the vanilla ice cream around its base and sides, then return to the freezer for 5 minutes. Next, spoon the orange ice cream into the hollow. Smooth the top of the ice cream bombe flat, cover with clingfilm and freeze for a few hours – overnight is ideal.
6To serve, turn the bombe out of its basin. I find this easiest by dipping the basin briefly in a bowl of just-boiled water, upturning it over a serving dish and shaking the basin and dish firmly. Give it a minute or two to slide out.
7Melt the chocolate in a bowl over a pan of barely simmering water, then trickle it over the bombe. Return it to the freezer for a couple of minutes for the chocolate to set crisply, and serve.
If you're feeling fancy, decorate with physalis dusted with gold lustre powder.