Line a baking sheet with parchment paper and set aside.

In a heavy-bottomed saucepan, combine butter, sugar, water, and salt; cook over medium heat, stirring constantly, until sugar dissolves.

Attach a candy thermometer and continue cooking without stirring until the mixture reaches 300-310°F (150-155°C), the hard crack stage; then remove from heat.

Stir in vanilla extract quickly, then immediately pour hot toffee onto the prepared baking sheet, spreading it evenly.

If using, sprinkle chocolate chips over the hot toffee and let sit for a few minutes until melted, then spread the melted chocolate evenly and sprinkle with chopped nuts.

Let cool completely at room temperature or in the refrigerator until firm, then break into pieces.