Line a 9×13-inch baking dish with parchment paper or a silicone mat for easy removal.

Combine unsalted butter, granulated sugar, salt, and water in a heavy-bottomed saucepan and heat over medium heat, stirring constantly, until it turns deep golden brown.

Remove from heat and stir in the vanilla extract.

Pour the toffee mixture onto the prepared baking sheet, spreading it evenly.

Sprinkle chocolate chips over the warm toffee and let sit for 2 minutes to melt.

Spread the melted chocolate over the toffee and sprinkle with chopped nuts, pressing down slightly.

Cool at room temperature for at least 2 hours, then break into pieces.