Grease a 9x9 pan. Sprinkle nuts into the bottom of the pan.
Bring butter, brown sugar, water, and salt to a boil over medium heat. Let boil, stirring about 12 minutes or until the mixture reaches 295°F on a candy thermometer.
Remove from heat and stir in vanilla. Pour into prepared pan and cool 30 minutes.
Prepare melted chocolate per notes below. Spread melted chocolate over toffee and sprinkle with additional nuts if desired.
Let cool 1 hour at room temperature then break into bite sized pieces.
