Place the milk in a medium saucepan. Heat over medium-low until it just begins to simmer.

While the milk is heating, whisk together the salt, sweetened condensed milk and egg yolks in a medium heat-proof bowl. Gradually pour the hot milk into the egg yolk mixture; whisking vigorously the entire time.

Return the mixture to the saucepan and cook over medium-low heat, stirring constantly, until the mixture thickens noticeably and coats a wooden spoon. (~5-6 minutes) (Note: Do not let the mixture boil.)

Remove mixture from heat and add the heavy cream, tequila, triple sec and lime juice; stir until well combined. Pour the mixture through a fine-mesh strainer into a clean container. Stir in the lime zest. Cover with plastic wrap, pressing the plastic directly onto the surface of the mixture, and refrigerate overnight.

Pour the ice cream mixture into an ice cream maker and freeze according to the manufacturer's directions.

(Optional) If you desire a harder ice cream, transfer the finished ice cream into the freezer for a couple of hours. Due to the alcohol, this ice cream won’t get super hard, but it will firm up after a couple of hours in the freezer.