In heavy saucepan, combine butter, sugar, water, and salt.
Cook to hard-crack stage (300°F) stirring constantly and watching carefully.
Immediately pour into Silpat or greased half-sheet pan.
Cool until hard.
Melt chocolate over hot, but not boiling water.
Spread over toffee; sprinkle with nuts, pressing them into chocolate.
Let stand 2-3 hours or chill 30 minutes.
Break into bite-size pieces.