Attach the thermometer to the saucepan. Scrape the seeds out of the split vanilla bean, then put the bean and its seeds in the bottom of the saucepan. Add the milk and 1/2 cup (100 g) of the sugar. Cook over medium heat, stirring occasionally, until the thermometer registers 170° F/77° C. Remove from heat, cover, and allow to steep for 30 minutes.
Remove the vanilla bean pod and return the pot to the burner. Warm over medium heat, stirring occasionally to prevent scorching, until the thermometer registers 170° F/77° C.
In a 3-quart bowl, whisk the cornstarch with the remaining 1/4 cup (50 g) sugar, then whisk in all the egg yolks until the mixture is foamy and slightly thickened.
Temper the egg yolks by gradually adding about half of the hot milk mixture, just one ladle at a time, whisking continuously. Whisk the tempered egg mixture into the saucepan and return to the burner. Cook over medium heat, stirring continuously with a wooden spoon or heatproof spatula until the thermometer registers 185° F/85° C. Do not allow the mixture to boil. Remove from heat and insert the immersion blender into the hot liquid. Blend (aka emulsify) until smooth. (Note: You can also use a whisk and vigorously whip the mixture, or use a blender.)
Make an ice bath in the large bowl and set the other 3-quart bowl on top. Add the heavy cream, rum, and nutmeg to the bowl and stir. Place a fine-mesh strainer on the rim of the bowl and carefully pour the custard through the strainer. You may need to stir it around and press through. Whisk the mixture together, and stir the custard about every 5 minutes until the mixture is cooled, about 30 minutes. Remove the bowl from the ice bath, dry the bottom, and cover with plastic wrap. Chill in the refrigerator for at least 8 hours or overnight.
About 30 minutes before churning, set the bowl in the freezer to get the mixture as cold as possible without starting to freeze. Pour the mixture into your ice cream maker and follow the manufacturer’s directions for processing.
Store the gelato in a plastic container. Press a layer of plastic wrap against the gelato and seal the lid on top. Freeze until fully hardened. Serve with a sprinkle of fresh nutmeg on top.
Prep Time includes churning time and overnight chilling time.