How to Make It
Set a medium bowl in a large bowl of ice water. In another medium bowl, whisk the egg yolks with the granulated sugar until pale, about 3 minutes.
In a medium saucepan, combine the half-and-half, liqueur and brown sugar and bring to a simmer, whisking until the sugar is completely dissolved. Whisk the hot cream mixture into the beaten egg yolks in a slow, steady stream.
Transfer the mixture to the saucepan and cook over moderately low heat, stirring constantly with a wooden spoon, until the custard is thick enough to lightly coat the back of the spoon, about 5 minutes; don’t let it boil. Pour the custard through a fine-mesh strainer into the bowl in the ice water. Whisk in the vanilla and the salt. Let the custard cool completely, stirring frequently. Refrigerate until very cold, at least 1 hour, preferably overnight.
Pour the custard into an ice cream maker and freeze according to the manufacturer’s instructions. Transfer the ice cream to a plastic container, cover and freeze until firm, at least 3 hours.