Melt the chocolate in a bowl, set over a pan of gently simmering water, making sure the bottom of the bowl is not touching the water. Remove from the heat once melted.
Pour the cream and milk into a large pan and bring nearly to the boil over a gentle heat.
Whisk the egg yolks and sugar until pale. Whisk a cupful of the hot cream mixture into the eggs and sugar. Pour this custard mixture back into the pan and cook over a low heat for 8–10 minutes, stirring constantly until thick. If there are any lumps, sieve into a clean pan.
Remove the custard from the heat and leave to cool for about 5 minutes. Take a cup of it and stir into the melted chocolate. Then stir all the chocolate back into the custard until fully incorporated.
Stir in the passion fruit powder and leave to cool completely but don’t refrigerate.
Cool, churn and freeze in an ice-cream maker, following the manufacturer’s instructions. If you don’t have an ice-cream maker, tip the ice cream into a bowl, then put the bowl in the freezer until the ice cream starts to freeze around the edges. Stir well, then repeat the process twice more until the ice cream is smooth.
Transfer to a sealable container for longer term storage.