Prepare an ice bath in a large bowl and have a strainer ready. Add sweetened condensed milk and salt to a medium bowl and set aside. Lightly beat 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. Remove from heat once you see steam rising from the surface and the mixture is completely heated through. Slowly, while whisking, add half of the warm cream mixture to the bowl with the yolks. Return the egg mixture to the saucepan, and cook over medium heat, stirring constantly with a wooden spoon. You will know the custard is thick enough when you see steam rise from the surface and the custard coats the spoon (2 - 3 minutes). Strain the custard into the bowl with the sweetened condensed milk, stirring to combine.

Place the bowl in the ice bath and stir occasionally to cool (be sure not to get any water in your custard mixture). Once cool, stir in the buttermilk. Using the tip of a knife, scrape the seeds from the vanilla pods and add them to the mixture along with the pods. Cover the bowl tightly and let chill in the refrigerator for 2 days.

Stir the mezcal into the chilled custard base, then strain the base into an ice cream maker. Freeze custard in an ice cream maker according to the manufacturer’s instructions. Transfer to an airtight container. Cover and freeze at least 2 hours before scooping.