Set a Bowl with water and silicone pastry brush next to your stove for easy access.
Set out a 9x9 metal pan. You can line it with parchment to help with removal of the toffee. If you would like nuts inside your toffee you can sprinkle a thin layer in the bottom of the pan at this time.
In a heavy sauce pan, combine butter, sugar, corn syrup, salt and 1/4 cup water. Cook over medium heat stirring frequently with wooden or silicone spoon until the butter melts and the sugar dissolves. .
Insert candy thermometer.
Then stir gently and only occasionally as the temperature nears 300° F and mixture begins to darken.
Occasionally brush the sides of the pan with he silicone brush and water to keep the sugar from crystalizing.
Once the mixture reaches 300° F, remove from heat and GENTLY add the vanilla and stir in GENTLY.
Using the silicone spatula, scrape the mixture into the prepared pan.
Tilt pan to evenly distribute the mixture. And let cool for 3 minutes.
Sprinkle chopped chocolate on top of warm/hot toffee and cover with a larger cookie sheet or foil to help retain the heat.
Let sit for a few minutes until melted and then, using the offset spatula, spread evenly.
Let cool for at least 3 hours. You can put it in the refrigerator to speed the cooling process.
Break pieces for a more rustic look or you can use a large chefs knife for a more finished look.
Store in and air tight container in the refrigerator.