Preheat the oven to 350°. Spread the hazelnuts on a rimmed baking sheet and bake for about 12 minutes, or until fragrant and browned; let cool. Transfer the hazelnuts to a kitchen towel and rub them together to remove the skins. Coarsely chop the hazelnuts.
In a large, heavy saucepan, combine the half-and-half and sugar and bring to a simmer over moderate heat, stirring to dissolve the sugar. Add the hazelnuts, cover and let steep off the heat for 30 minutes.
In a large stainless-steel bowl, whisk the egg yolks. Very gradually whisk in 1/2 cup of the hot half-and-half, then gradually whisk in the rest. Pour the mixture into the saucepan and cook over very low heat, stirring constantly, until the custard thickens, about 10 minutes. The custard should be thick enough to coat the back of a spoon.
Strain the custard into a large stainless-steel bowl set in an ice water bath and discard the hazelnuts. Stir occasionally until the custard is thoroughly chilled. Freeze the custard in an ice cream make, in batches, according to the manufacturer's instructions. Store the ice cream in airtight containers in the freezer for up to 3 days.