Gianduia Gelato

Pulse hazelnuts with sugar and salt in a food processor until finely ground. Transfer to a heavy medium saucepan with milk and bring just to a boil, stirring occasionally. Remove from heat and let steep, covered, 20 minutes. Strain mixture through a fine-mesh sieve into a bowl, then return to cleaned saucepan. Put 1/4 cup milk mixture in a small bowl and whisk in cornstarch. Return to saucepan, then boil over medium heat 2 minutes, whisking often (mixture will be thick). Remove from heat and add chocolate, stirring until melted and incorporated. Chill, stirring occasionally, until very cold, 3 to 6 hours. Freeze mixture in ice cream maker. Transfer to an airtight container and put in freezer to firm up.