In a medium sauce pot, combine milk and cream. Set over medium heat and bring to a boil.
In a large bowl, whisk together brown sugar, egg yolks, and salt. Once milk begins to boil, lower heat. Temper hot milk into yolk mixture about ¼ cup at a time, whisking to distribute heat. Once all the milk has been added, transfer everything back into the pot.
Stir over medium-low heat until mixture reaches nappe consistency, or 180°F on a kitchen thermometer. Let sit at 180-185 degrees F for 3-5 minutes to pasteurize eggs.
Transfer to an airtight container and let mature in the fridge overnight.
Freeze mixture in ice cream maker according to your unit's instructions. Once the volume has increased and mixture has reached a soft-serve consistency, add whiskey. Transfer mixture to a freezer safe container. Let rest in the freezer for at least one hour before serving.