Combine the two chocolates and set aside. Set a small bowl of water and a pastry brush next to the stove. In a heavy saucepan fitted with a candy thermometer, cook the butter, sugar, water, corn syrup, and salt over medium heat. Stir frequently with a wooden spoon until the butter melts and the sugar dissolves; then stir gently and only occasionally as the mixture approaches 300° 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° F (this will take about 18 to 20 minutes), remove the pan from the heat, carefully add the vanilla, and stir it in. With a heatproof rubber spatula, scrape the mixture into a metal 9x11-inch baking pan set on a rack. Tilt the pan until the toffee covers the bottom of the pan evenly. Let 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 to 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. To help the chocolate set faster on a warm day, refrigerate the candy.
Photo: Scott Phillips