Line a 9x13-inch baking pan with parchment paper and lightly butter the parchment. Sprinkle half of the chopped nuts evenly over the bottom.
In a heavy-bottomed saucepan, combine butter, sugar, water, and salt. Cook over medium heat, stirring constantly until the butter melts and sugar dissolves.
Attach a candy thermometer to the side of the pan. Continue cooking, stirring frequently, until the mixture reaches 300°F (hard crack stage). This will take about 12-15 minutes and the mixture will turn a golden amber color.
Remove from heat immediately and stir in the vanilla extract. Carefully pour the hot toffee over the nuts in the prepared pan, spreading quickly with a buttered spatula.
Let cool for 1-2 minutes, then sprinkle the chocolate chips evenly over the hot toffee. Wait 2-3 minutes for the chips to soften, then spread the melted chocolate in an even layer using an offset spatula.
Immediately sprinkle the remaining chopped nuts over the melted chocolate, pressing gently so they adhere.
Refrigerate for at least 1 hour until completely set. Break into irregular pieces by hand or cut into squares with a sharp knife.
Store in an airtight container with parchment paper between layers for up to 3 weeks at room temperature, or refrigerate for up to 1 month.