Prepare a baking sheet by lining it with parchment paper.
In a medium saucepan, combine butter, sugar, water, and salt.
Heat the mixture over medium heat, stirring constantly until the sugar dissolves.
Bring the mixture to a boil and stop stirring.
Use a candy thermometer to monitor the temperature, aiming for 300°F (hard crack stage).
Once the desired temperature is reached, remove from heat and quickly stir in vanilla extract.
If using nuts, fold them into the mixture at this stage.
Pour the hot toffee onto the prepared baking sheet, spreading it evenly.
If desired, sprinkle chocolate chips on top and let them melt before spreading evenly.
Allow the toffee to cool completely before breaking it into pieces.