Chocolate Peanut Toffee

Ingredients
* 4 sticks (1 pound) unsalted butter, cut into pieces
* 2 cups sugar
* 1/4 teaspoon salt
* 4 cups whole cocktail peanuts plus 1 cup chopped (1 pound 10 ounces)
* 7 to 8 ounces 70%-cacao bittersweet chocolate, finely chopped
* Equipment: a 15- by 10- by 1-inch baking pan; a candy thermometer; a metal offset spatula

Preparation
Butter baking pan and put on a heatproof surface.

Bring butter, sugar, and salt to a boil in a 4- to 5-quart heavy pot over medium-high heat, whisking until smooth, then boil, stirring occasionally, until mixture is deep golden and registers 300