1 pound butter (see note), plus more for preparing baking sheets
1 cup granulated sugar
1 cup brown sugar
1 tablespoon corn syrup
2 1/2 cups raw hazelnuts, coarsely chopped
2 teaspoons vanilla extract
1/4 teaspoon salt
16 ounces semisweet chocolate (either chips or squares that have been broken into chunks)
2 cups chopped, dark-roasted and peeled hazelnuts

Grease 1 large, rimmed baking sheet (12 by 17 inches) or two smaller ones (10 by 15 inches or smaller) with butter. If you're lucky enough to have a kitchen equipped with a marble slab, then use it instead of baking sheets. Grease it the same way.

In a large, heavy-bottomed pot over medium heat on a wide-diameter burner, melt the 1 pound butter. While butter is melting, combine the granulated sugar, brown sugar and corn syrup. Add this mixture to melted butter, along with 1/4 cup water. When sugars have dissolved, increase burner setting to medium high, stirring with a flat-edged wooden spoon. Attach a candy thermometer to pot and cook mixture, stirring continuously to keep it from scorching. When temperature reaches 240 F, add the raw hazelnuts and continue stirring. At 260 F, reduce burner setting to medium-low and continue cooking and stirring occasionally, until toffee reaches 300 F ("hard crack").

Remove pot from burner and stir in the vanilla and salt, stirring quickly because toffee will become quite thick at this point. Scrape it out onto prepared baking sheet(s) or marble slab, spreading it out to a thickness of about 1/4 to 1/2 inch. A silicone spatula is best for spreading sticky toffee.

Allow toffee about 5 minutes to set up, then spread the chocolate pieces evenly over surface. Heat from toffee will melt it. While it's still soft, gently spread chocolate evenly over surface of toffee and then sprinkle it with the roasted and chopped hazelnuts.

At this point, you can refrigerate toffee until it is cold and hard. Or you can just let it cool to room temperature over several hours or overnight. Once toffee is very firm, it can be broken into smaller, irregular-sized pieces. I However, if you want pieces to break into uniform shapes, just score toffee slab appropriately before layering on chocolate.

Makes 4 1/4 pounds of toffee (100 pieces measuring 1 by 2 inches).

NOTE: I use salted butter, but if you use unsalted butter, increase amount of salt to 1 teaspoon.