Line a 9×13-inch baking pan with parchment paper and lightly grease.

Combine butter, sugar, water, and salt in a medium saucepan over medium heat until melted and bubbling.

Bring mixture to 300°F (149°C) hard crack stage, stirring occasionally.

Remove from heat and stir in vanilla extract.

Pour hot toffee into prepared pan and spread evenly to 1/4-inch thickness.

Sprinkle chopped chocolate over hot toffee, wait one minute, then spread until smooth.

Add toasted nuts on top, pressing lightly to adhere.

Allow toffee to cool completely, then break into pieces. Store in an airtight container.