Put the water into a small heavy-based saucepan and add the sugar, stirring it well to help the sugar dissolve. Place a jam thermometer in the pan, then bring it up to the boil and boil vigorously until the mixture reaches 120 degrees, or hard-ball stage. This will take about 6 minutes.

Using an electric mixer, beat the egg yolks until thick and creamy. As soon as the sugar syrup reaches 120 degrees, turn the mixer to its highest setting and pour a thin, slow stream of the just-boiled syrup into the egg yolks as you whisk. The egg yolks will gradually expand into a thick pale creamy mousse – keep whisking until it is really thick. Set aside.

Break the chocolate into a large bowl and set it over a large pan of just-boiled water. Stir occasionally until the chocolate has melted. You may need to replace the hot water once.

Meanwhile, put the milk and cream in a saucepan and bring up to just below boiling point. It should be just a little hotter than the chocolate. Using a wooden spoon, stir the hot milk and cream into the chocolate bit by bit. Once all the chocolate is melted into all the milk, transfer to a jug and chill in the fridge.

If using the nori sheets, take a pair of scissors and finely cut the nori into tiny fragments. Otherwise, just add the dried nori flakes to the chocolate once it is tepid, then whisk the mixture into the egg mousse. Some of the nori will stick to the whisk, but scrape it off and return it to the mixture.

Pour into your ice-cream machine and churn according to the manufacturer’s instructions. Alternatively, pour into a shallow container, freeze and, after the first hour, beat again in the electric mixer until nice and smooth. You might need to repeat this at least once until you have a smooth texture. Freeze until set.