Pour most of the coconut milk into a saucepan, reserving a splash in a small bowl for later. Add the sugar, cocoa, chocolate, sea salt flakes and vanilla to the pan. Heat gently for 10 mins until the sugar has dissolved and the chocolate has melted.
Mix the cornflour with the reserved coconut milk until smooth. Pour into the hot milk mixture and cook for another 5-10 mins, stirring constantly, until the mixture thickens to a pourable custard consistency. Strain into a bowl then cover and leave to cool to room temperature.
Pour the cooled custard into an ice cream maker and churn for 20-30 mins until you have a soft-scoop ice cream. Transfer to a freezerproof sealable container and freeze for up to three months, or until ready to serve. If you don’t have an ice cream maker, pour the mixture into a wide-based plastic tub or dish. Freeze for 3 hrs, stirring every 20 mins to break up any large ice crystals, until you have a soft ice cream. Transfer to a freezerproof sealable container and freeze until ready to serve. Leave to stand for 10 mins at room temperature before scooping into bowls or cones.
