1. To make the ice cream: Put the ginger, sugar, salt, and 1 cup water in a small saucepan, stir well, and bring to a boil over medium-high heat. Turn the heat to low and simmer, stirring occasionally, until the mixture becomes syrupy and very thick, about 5 minutes. Set aside to cool.

2. Whisk the egg yolks in a medium mixing bowl until broken; set aside. Put the milk and 1 cup of the cream in a medium saucepan and warm over medium heat, stirring occasionally, until bubbles begin to form around the edges. Remove from the heat and pour ½ cup of the warm milk mixture onto the yolks in a slow, steady stream, whisking constantly. Transfer the yolk mixture back to the saucepan. Set over low heat and cook, stirring constantly, until the mixture is thick enough to coat the back of a wooden spoon and registers 165°F, about 5 minutes. Remove from the heat and stir in the butter until the butter melts.

3. Strain the cream mixture through a fine-mesh sieve into a large mixing bowl and stir in the remaining ½ cup cream and rum. Strain the ginger syrup into the same bowl, pressing on the ginger to extract as much liquid as possible. Set over a larger bowl of ice and water and stir occasionally until cold to the touch, about 40°F. Alternatively, cover and refrigerate until cold.

4. Transfer the mixture to your ice cream maker and freeze following the manufacturer’s instructions. After churning, stir in the raisins and transfer to a freezer container. The ice cream will be soft when just churned but will firm up in the freezer. It is best fresh, but it will keep in an airtight container in the freezer for up to 3 days.