Chill the ice cream bowl for at least 24 hours or until solid. You should not hear any water sloshing around when you shake the bowl.
Stirring often, heat the milk in a small sauce pan over low heat to just before it starts to boil.
Remove the pan from the heat and add the rosemary sprigs, and stir. Allow them to steep for 30-45 minutes.
Remove the rosemary sprigs and any rosemary pieces from the milk. Chill the rosemary milk for 30 minutes. If milk solids form on the top you can skim them off, but it’s not necessary.
In a large bowl, combine the rosemary milk, heavy cream, granulated sugar, vanilla extract, and kosher salt.
Whisk for 2-3 minutes, until the sugar is dissolved into the mixture. You should not feel any grit if you rub the mixture together between two fingers.
Churn the ice cream mixture in the bowl of your ice cream machine according to your manufacturer’s instructions (mine took about 25 minutes), until it is thick and the consistency of soft-serve ice cream.
Spoon the rosemary ice cream into a shallow container or a loaf pan.
Press plastic wrap or wax paper to the surface of the ice cream. Freeze for at least 4 hours, or until solid.
Let sit at room temperature for about 5 minutes before serving.