In a saucepan, pour in ½ cup / 120 ml (4.1 fl oz) of the soymilk and whisk in the arrowroot until completely dissolved.
Add the remaining 1 cup / 240 ml of soymilk, pumpkin puree, maple syrup, vanilla, cinnamon, and nutmeg.
Whisk everything together over medium-high heat.
Bring the mixture to a low boil.
Once it begins to bubble, whisk constantly until it thickens. This should happen very quickly!
Remove the saucepan from the heat. If using, stir in the rum.
Pour into mugs, garnish with a sprinkle of freshly grated nutmeg, and serve hot.