Combine the cream, milk and 1/2 cup of the sugar in a medium heavy-bottomed saucepan and bring to the boil over medium heat, stirring occasionally. Meanwhile, in a large bowl, whisk the yolks, the remaining 1/2 cup sugar and the spices until thick.
Gradually whisk the hot cream mixture into the yolk mixture. Return the mixture to the saucepan and cook over medium heat, stirring constantly with a wooden spoon, until the custard thickens, about 3 to 5 minutes; do not let the custard boil. When you draw your finger across the back of the spoon, it should leave a track. Strain the custard into a large clean bowl. Add the pumpkin and vanilla extract and whisk to blend. Let cool to room temperature.
Freeze the custard in an ice cream maker following the manufacturer's instructions. When the ice cream is almost ready, add the dark rum and process until frozen. Pack into a freezer container and freeze to firm and ripen. The ice cream should be ready to scoop in about two hours, but it will keep in the freezer for two weeks.