1. In a 350° oven, bake 1 cup hazelnuts in an 8- or 9-inch square pan until nuts are golden beneath skins, about 12 minutes. Pour nuts onto a towel and rub to remove loose skins. Discard skins and let nuts cool. Coarsely chop 1/2 cup nuts.
2. Prepare Deep, Dark Chocolate Sauce. Pour sauce into a blender and add whole hazelnuts, Frangelico, and corn syrup. Whirl until chocolate truffle sauce is very smoothly puréed.
3. Return chocolate truffle sauce to pan and nest pan in ice water. Stir often until sauce is cold, about 20 minutes. Scrape into chilled bowl (see notes) and freeze while making ice cream.
6. In a 2- to 3-quart pan, combine sugar and half-and-half. Stir over high heat until bubbles form at pan edge (scalding, about 180°), 5 to 8 minutes.
7. In a small bowl, whisk eggs to blend. Then whisk about 1/2 cup of the hot cream mixture into eggs. Return egg mixture to pan and stir over medium-low heat with a flexible spatula-scraper, scraping pan bottom and sides thoroughly for even cooking, until custard thickly coats a metal spoon (about 190°), 8 to 10 minutes.
8. Add vanilla to custard.
9. At once, nest pan in ice water and stir custard often until mixture is cold, 10 to 15 minutes.
10. Pour cold custard through a fine strainer into a bowl, then pour mixture into an ice cream maker (1-qt. or larger capacity), or strain directly into the maker; discard residue. Add reserved chopped nuts to cold custard in ice cream maker. Freeze according to manufacturer's directions until mixture is firm enough to scoop, dasher is hard to turn, or machine stops.
11. When ice cream is frozen, remove bowl with truffle sauce from freezer. Stir sauce briefly to soften, then quickly scoop ice cream into bowl. With a flexible spatula-scraper, swirl chocolate through ice cream, using 4 or 5 strokes.
12. Return to freezer for at least 3 hours to firm. Package airtight if storing longer (see below).
Firming and Storing Ice Cream:
To get frozen desserts hard enough to scoop onto a cone, or to store them, transfer when frozen to an airtight container and put in the freezer at least 3 hours or up to 1 week.
If freezing with ice and salt, leave the frozen dessert in ice and salt up to 3 hours.
For best flavor and texture, serve frozen desserts within a week. On longer standing, icy crystals develop.
Nutritional analysis per 1/2 cup.