2. In a saucepan, melt the butter over medium heat.
3. Stir in the sugar, vanilla extract, and salt. Continue to cook, stirring constantly, until the mixture reaches 300°F (150°C) on a candy thermometer.
4. Pour the mixture onto the prepared baking sheet and spread it evenly.
5. Sprinkle the chocolate chips over the hot toffee. Let them sit for a minute to melt, then spread the chocolate evenly over the toffee.
6. If using, sprinkle the chopped nuts over the melted chocolate.
7. Let the toffee cool completely, then break it into pieces.