Combine the two chocolates and set aside.
Set a small bowl of water and a pastry brush next to the stove.
In a heavy-based saucepan fitted with a candy thermometer, cook the butter, sugar, ¼ cup 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 degrees Fahrenheit 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 Fahrenheit (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 cooling rack. Tilt the pan until the toffee covers the bottom of the pan evenly. Let cool for 2 minutes.
Sprinkle the toffee with the chopped chocolate and cover the pan with another baking pan (to keep things warm and help the chocolate melt) for a few minutes.
Smooth the melted 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.