Mix the milk, beaten eggs,vanilla extract, sugar (and optional locust bean gum) together. If you are using locust bean gum, it's a good idea to mix it into the sugar before you add it to the wet ingredients to avoid clumping.

Add the mixture to a saucepan and heat gently, stirring vigorously all the time until it starts to thicken. This will take 8 - 10 minutes. Do not overheat or the mixture will curdle...

If you have a kitchen thermometer heat to 85°C / 185°F. If you don't have a thermometer heat until the mixture coats the back of a spoon and holds a line drawn through it with your finger.

As soon as the mixture hits 85°C / 185°F, remove it from the heat, and continue to stir while you submerge the bottom of the pan in cold water. This will prevent any residual heat from curdling the custard.

Rip up the springs of mint and lemon balm and add them to the cooling custard. Give them a good muddle.

Let the mixture cool to room temperature and then transfer it to the fridge. Leave for a least an hour, preferably two. Even better overnight. Taste the mixture to make sure you're happy.

Stir in the cream and then pour the mixture into your ice cream machine through a sieve, pressing the mint and lemon balm to make sure you get all the flavor out.

Churn the mixture until it starts to come away from the sides of the bowl and looks like whipped cream.

You can eat it directly from the machine (but it will melt quickly). Or transfer it to a plastic freezer container, cover with cling film or grease-proof paper (to prevent ice crystals forming on the surface) and then a lid and leave for a least a couple of hours.

Remove from the freezer and if it's frozen solid, let it soften for 15 or 20 minutes before serving.