Preheat the oven to 350°F.
Spread the hazelnuts on a baking sheet. Bake for about 10 minutes, stirring occasionally until the skins are cracked and the nuts (under the skins) are deeply golden brown. Place the nuts in a towel and rub them to remove most of the skins. Cool completely.
Combine the cream and 1 cup of the milk in a large heavy saucepan. Scrape the vanilla seeds into the cream mixture and add the bean and the hazelnuts. Bring to a gentle boil over medium heat.
Whisk the yolks and sugar in a medium bowl. In a slow, steady stream, whisk 1 cup of the hot cream mixture into the egg mixture. Gradually whisk the egg mixture back into the saucepan. Stir constantly over medium-low heat until the mixture thickens enough to coat the back of a spoon and reaches 170°F on an instant-read thermometer, about 5 minutes. Strain through a fine-mesh sieve into a heatproof medium bowl.
Discard the vanilla bean, and rinse the nuts in the strainer under cold running water; pat dry on paper towels. On a work surface, crush the nuts with a meat mallet or rolling pin, and stir into the custard. Stir in the remaining 1 cup milk and the liqueur. Cover with plastic wrap, pressing the wrap aganist the surface of the custard to prevent a skin from forming. Refrigerate until well chilled, at least 2 hours.
Pour the chilled custard into an ice cream machine. Churn according to the manufacturer’s instructions. Pack into an airtight container and freeze until ready to serve.
Yield: 1 1/2 quarts