Slit the vanilla beans lengthwise and scrape out the seeds. In a small saucepan, whisk together the milk, sugar, salt and vanilla seeds or extract. Bring almost to a boil and barely simmer for a few minutes, just until the sugar is dissolved. Let cool and then chill in the fridge for at least 4 hours. (You can skip the stove step and simply pour the mixture straight into your ice cream maker once you have whisked the sugar into the milk. I have done it both ways several times and they were both excellent. There is a slightly creamier texture to the simmered version, but it is not a big difference. I make both versions and it really just depends on how much time I have.)
Pour the mixture into your ice cream maker and freeze according to manufacturer's directions. When the machine is finished, pour into an airtight container and freeze until ready to eat.
This is a perfect soft serve straight out of the machine. I froze it for a couple hours though before serving it. Two days later it was frozen firm, but still scoop-able straight out of the freezer.