Preheat oven to 350 degrees. Toast the chopped walnuts on a baking sheet for 6-8 minutes. Reserve.
Line a baking sheet with heavy foil, folded with a 1-inch high edge. Reserve.
In a heavy bottom saucepan, cook the butter, sugar, water and salt over medium heat until the temperature reaches 305 degrees (hard-crack stage), without stirring. This will take about 15 minutes. If the sugar mixture begins browning unevenly give the pan a swirl. Keep a close eye on it once it reaches 290 degrees as the temperature will increase quickly.
Remove the pan from the heat and immediately stir in the vanilla extract. Pour the mixture onto the foil lined baking sheet and allow it to spread. Cool at room temperature for 45 minutes.
Melt chocolate according to package instructions. Spread melted chocolate over cooled toffee, then sprinkle with walnuts. Let set at room temperature until the chocolate has set. Break toffee into pieces. Store covered at room temperature.
The total time required is comprised of two 45 minute resting periods.