In a medium saucepan, heat the milk, sugar, 1 cup (250 ml) heavy cream, salt, and mint.
Once the mixture is hot and steaming, remove from heat, cover and let stand for an hour to infuse.
Remove the mint with a strainer, then press down with a spatula firmly to extract as much mint flavour and colour as possible. (You can also use well-washed hands to do it as well, making sure the mixture isn’t too hot to safely handle). Squeeze every last drop! Once the flavour is squeezed out, throw out the mint.
Pour the remaining heavy cream into a large bowl and set a strainer over the top.
Rewarm the infused milk. In a separate bowl, whisk together the egg yolks, then slowly pour some of the warm mint mixture into the yolks, whisking constantly, then scrape the warmed yolks back into the saucepan.
Cook the custard, stirring constantly with a heatproof spatula, until the mixture thickens and coats the spatula. If using a thermometer, it should read around 77ºC.
Immediately strain the mixture into the cream, then stir the mixture over an ice bath until cool.
Refrigerate the mixture thoroughly, preferably overnight. Add food colouring if you wish. Freeze it in your ice cream maker according to the manufacturer’s instructions.
While the mixture is freezing, melt the chocolate in a small bowl over a pot of simmering water, or in a microwave oven on low power, stirring until smooth. Place a storage container in the freezer.
When the ice cream in the machine is ready, scribble the chocolate into a container. Spoon some ice cream into the container, allow the chocolate to set (about 20 seconds), then using a big metal spoon, mix the chocolate into the ice cream, breaking it up as you mix. Repeat with the remaining chocolate and ice cream. Cover and freeze until firm.