Step 1Heat the oven to 350 degrees. Spread the nuts on a baking sheet and toast them until they're lightly browned, about 5 to 7 minutes. Finely chop the nuts.
Step 2Line a 9-inch square baking pan with foil, grease the foil with butter and set it aside.
Step 3Clip a candy thermometer to a heavy 2 1/2-quart saucepan. Bring the sugar, butter, water and salt to a boil over medium-high heat, stirring often, about 5 minutes. Continue boiling, uncovered, not stirring the mixture but shaking the saucepan occasionally to distribute the heat as the mixture turns amber in about 9 minutes, then darker amber as it registers 305 degrees on the thermometer (hard-crack stage), about 3 minutes longer. Stay close by; the thermometer will reach 305 degrees quickly. Remove the pan from the heat; stir in the vanilla.
Step 4Carefully pour the hot mixture into the prepared pan. Let it cool to room temperature until hard, about 30 minutes.
Step 5Melt the milk and bittersweet chocolates in a double boiler or at 50% power in the microwave. Mix them together, then spread the chocolate evenly over the toffee using an offset spatula. Immediately sprinkle the toffee with the nuts. Refrigerate for 30 minutes, then remove the toffee from the pan and break it into pieces.