Line a 12-x-17-inch rimmed baking sheet with non-stick aluminum foil. While the candy is cooking, place the baking sheet on two of the stove burners on low heat.
Melt the butter in a medium saucepan. Remove the pan from the heat, and using a silicon spatula, stir in the sugar, corn syrup and water. Position a candy thermometer in the pan so that the bulb doesn’t touch the bottom of the pan. Cook the mixture over medium-high heat, stirring occasionally, until it reaches 300° F, a process that can take 10 minutes or so, depending on your pan and the heat under it. At this point, the toffee will be caramel colored and bubbling. Take the pan off the heat, remove the thermometer and stir in the salt and 1 cup of the nuts.
Cautiously pour the hot, sticky caramel into the heated lined baking sheet. Use a silicon spatula to spread the toffee over the bottom of the pan. It’s okay if you have a ragged oblong or an odd shape. Don’t feel as if you have to push the toffee all the way to the edges of the pan, but you do have to work fast because the toffee hardens quickly.
After the toffee has cooled for 10 minutes, sprinkle the chocolate chips over its surface. Let the chocolate chips sit for several minutes until they are melted. Using a silicon spatula or small offset spatula, spread the melted chocolate over the toffee. Sprinkle the remaining nuts over the chocolate; lightly press the nuts into the chocolate.
Refrigerate the candy until the chocolate is set, about 30 minutes, before breaking it up or cutting it. Yield: 12 servings. (See Note 3)