5 oz hazelnuts (1 cup)
2 1/2 cups whole milk
1/3 cup heavy cream
4 large egg yolks
1/4 cup plus 2 tablespoons sugar
Toast hazelnuts and rub off any skins in a kitchen towel, then cool completely. Pulse with 1/4 cup sugar and a pinch of salt in a food processor until finely chopped. Transfer to a 3-quart heavy saucepan and add milk and heavy cream. Bring just to a simmer over moderate heat, stirring occasionally, then remove from heat and let steep, covered, 1 hour. Pour mixture through a fine-mesh sieve into a bowl, pressing hard on and discarding solids. Beat together egg yolks and sugar with an electric mixer at medium speed until thick and pale, 2 to 3 minutes. Beat in milk mixture and transfer to cleaned saucepan. Cook custard over moderately low heat, stirring constantly, until an instant-read thermometer registers 175F (do not let boil). Immediately pour custard through cleaned sieve into a metal bowl, then set bowl in a larger bowl of ice and cold water and cool, stirring occasionally. Chill custard, covered, until cold. Freeze in an ice cream maker, then transfer to an airtight container and put in freezer to harden.