Preheat the oven to 400 degrees F.
Line a baking sheet with parchment paper. Place the hazelnuts on the baking sheet and roast until golden, about 4 minutes. Remove from the oven and cool completely. Roughly chop the nuts and set aside.
In a saucepan, over medium heat, combine the half-and-half and sugar. Bring to a simmer. In a small mixing bowl, whisk the egg yolks until smooth. Add 1 cup of the hot liquid to the egg yolks and whisk until smooth. Add the yolk mixture to the saucepan of liquid and whisk until incorporated. Bring the liquid back to simmer and continue to cook for 4 to 6 minutes or until the mixture coats the back of a spoon.
Pour the mixture into a glass bowl and place a piece of plastic wrap directly on top of the mixture. This will prevent a skin from forming while cooling. Cool the mixture completely. Stir in the nuts. Process the mixture in an ice cream machine.