Prepare an ice bath in the sink or in a large heatproof bowl.

In a medium saucepan, combine the sugar, skim milk powder, and milk. Stir with a hand mixer or whisk until smooth. Make sure the skim milk powder is wholly dissolved into the mixture and that no lumps remain (any remaining sugar granules will dissolve over the heat). Stir in the cream.

Clip a candy thermometer to the saucepan and set the pan over medium heat. Cook, stirring often with a rubber spatula and scraping the bottom of the pan to prevent sticking and burning, until the mixture reaches 110°F (45°C), 5 to 10 minutes. Remove the pan from the heat.

Place the egg yolks in a medium bowl. While whisking, slowly pour ½ cup (120 ml) of the hot milk mixture into the egg yolks to temper them. Continue to whisk slowly until the mixture is an even color and consistency, then whisk the egg-yolk mixture back into the remaining milk mixture.

Return the pan to the stovetop over medium heat and continue cooking the mixture, stirring often, until it reaches 165°F (75°C), 5 to 10 minutes more.

Remove the pan from the heat and add the rum, vanilla, nutmeg, and cinnamon. Whisk to combine. Transfer the pan to the prepared ice bath and let cool for 15 to 20 minutes, stirring occasionally. Pour the ice cream base through a wire-mesh strainer into a storage container and place in the refrigerator for 1 to 2 hours, or until completely cool.

Transfer the cooled base to an ice cream maker and churn it according to the manufacturer's instructions.

Transfer the ice cream to a storage container. Serve immediately or harden in your freezer for 8 to 12 hours for a more scoopable ice cream.