2 sticks (1/2 pound) unsalted butter, cut into small pieces
1 1/4 cups light corn syrup
1 cup dark corn syrup
1 cup heavy whipping cream
3/4 cup firmly packed light brown sugar
1/2 cup granulated sugar
1 teaspoon pure vanilla extract
1 1/2 cups toasted and coarsely chopped walnuts or unskinned almonds
Generously butter a 13-by-9-inch metal baking pan. In a large heavy saucepan, combine the butter, light and dark corn syrups, cream, brown sugar and granulated sugar. Cook over moderate heat, stirring to dissolve the sugar.
Raise the heat to moderately high and place a candy thermometer in the pan. Bring to a boil. Using a pastry brush dipped in warm water, brush down the side of the pan twice to prevent crystallization. Continue to cook, stirring constantly, until the temperature registers 248°, about 20 minutes.
Remove the pan from the heat and quickly stir in the vanilla and the nuts. Pour the caramel into the prepared pan and let cool to room temperature on a rack, at least 2 hours.
Remove the caramel from the pan. Oil the blade of a chef's knife. Cut the caramel crosswise into 13 strips, then cut each strip into 9 pieces. Arrange the caramels, without touching one another, between sheets of wax paper, or wrap them individually in wax paper.
Store the caramels in an airtight container at room temperature for up to 2 weeks.