Set it out in a bowl, and watch how quickly this irresistible brittle disappears.
Brush a 9-by-13-inch rimmed baking sheet with butter. Sprinkle the chopped nuts over the baking sheet; set aside.
Combine the sugar, corn syrup, water, and butter in a medium saucepan. Cook over medium heat, stirring occasionally until the sugar has dissolved. Using a pastry brush dipped in water, brush away any sugar crystals on the side of the pan to prevent recrystallization. Without stirring, continue to cook until the mixture reaches the hard-crack stage on a candy thermometer (300 degrees).
Remove the pan from the heat. Add the chocolate while stirring constantly.
Pour the mixture onto the prepared rimmed baking sheet. Spread quickly into a thin layer using an oiled metal spatula. Let cool completely. Break into pieces, and store in an airtight container at room temperature for up to 1 month.