In a medium saucepan, bring milk and chopped hazelnuts to a gentle boil. Turn off heat. Steep hazelnuts in milk for 2 hours.
Press the hazelnuts through a fine-mesh sieve with the back of a wooden spoon, pressing as much of the milk out as possible. Discard hazelnuts.
Combine egg yolks, pinch of salt, and sugar in the bowl of a mixer and beat at medium-high speed until very thick and pale yellow, 3-5 minutes.
Meanwhile return hazelnut milk mixture to a gentle simmer over medium heat.
To temper the egg yolks, add half of the milk to the egg mixture and whisk until well blended. Stir in remaining milk and cook over low heat, whisking constantly until mixture is thick enough to coat a spoon.
Remove from heat and immediately stir in heavy cream. Strain mixture through a sieve or strainer into a bowl set over an ice bath to chill. Stir in hazelnut liqueur.
Follow your ice cream manufacturer's instructions to churn ice cream. Stir in the crushed hazelnuts after ice cream has churned but before it's completely firm. (Usually about 5 minutes before ice cream machine is done).
Pack ice cream in airtight container and freeze several hours or overnight to cure