Add sweetened condensed milk and salt to a medium bowl and set aside.
Whisk egg yolks in another medium bowl and set aside.
Heat heavy cream, milk and sugar in a saucepan over medium heat, stirring to dissolve the sugar. As the liquid begins to simmer, remove from heat. Slowly pour a cup of the heated milk into the eggs and whisk constantly to ensure eggs don't start cooking. Pour egg mixture back into the original saucepan and continue cooking on medium heat until the base thickens and can coat the back of a spoon. Do not let boil.
Strain the heated liquid into the bowl with the sweetened condensed milk and stir.
Allow the custard base to cool then stir in the buttermilk and vanilla essence and add chilli flakes. Cover the bowl with cling wrap and let chill in the refrigerator for 2 days, yes 2 days. It really infuses the flavours.
Stir the magical Mezcal into the chilled custard base. Add the chilled custard base to your ice cream maker and churn to your manufacturer's instructions.
As the ice cream churns, melt the chocolate on a low heat in a saucepan. Once melted, allow to cool completely.
Once churning is complete add the ice cream to an airtight container. As you layer in the ice cream, pour in streaks of melted chocolate.
Allow the ice cream to freeze at least 3 hours before serving.
Notes
Make sure you melt the chocolate early enough so that it has cooled completely before adding to the ice cream. Any heat added to the ice cream will create ice crystals and trust me, the Mezcal worm frowns on ice crystals.