1. Bring milk and salt to a boil in a 2- to 3-quart heavy saucepan over moderate heat. Remove from heat.
2. Whisk together yolks and sugar in a bowl, then gradually add 1/4 cup hot milk, whisking. Add yolk mixture to milk remaining in pan in a slow stream, whisking, and cook over low heat, stirring constantly with a wooden spoon, until mixture is slightly thickened, coats back of spoon, and registers 80°C on thermometer, 3 to 5 minutes. Immediately pour through a fine-mesh sieve set into a clean bowl and stir in cream, rum, vanilla, and nutmeg. Chill custard, covered, until cold, at least 2 hours.
3. Freeze in ice cream maker, then transfer to an airtight container and put in freezer to harden, at least 2 hours. Soften slightly in refrigerator before serving, about 20 minutes.
· Custard can be chilled up to 1 day. · Ice cream can be made 1 week ahead.