6 oz. semisweet chocolate, coarsely chopped
6 oz. bittersweet chocolate, coarsely chopped
8 oz. (1 cup) butter
1 cup white sugar
1 tsp light corn syrup
1/2 tsp salt
1 tsp vanilla
1/2 cup finely chopped toasted pecans
Combine the chocolates and set aside. Set a small bowl of water and a pastry brush next to the stove. In a saucepan fitted with a candy thermometer, cook the butter, sugar, 1/4 cup water, corn syrup, and salt over medium heat.
Stir frequently with a spoon until the butter melts and the sugar dissolves; then stir gently and only occasionally as the mixture approaches 300 degrees F and begins to darken.
Brush the sides of the pan down with a little water every once in a while to keep the sugar from crystallizing. When the mixture reaches 300 degrees F (this will take about 18-20 minutes), remove the pan from the heat, carefully add the vanilla, and stir in it. With a heatproof rubber spatula, scrape the mixture into a metal 9×11-inch baking pan set on a rack. Tilt the pan until the toffee covers the bottom of the pan evenly. Let this cool for 2 minutes.
Sprinkle with the chopped chocolate and let it melt for a few minutes (cover with another baking pan to help it melt). Smooth the chocolate with a spatula (use a narrow offset spatula if you have one) and sprinkle on the pecans. Let cool completely (3-4 hours) and then break or chop into pieces; use a metal spatula or a blunt knife to pry the toffee out of the pan.