In heavy saucepan, combine butter, sugar, water, and salt. Cook to hard-crack stage (300°F) stirring constantly and watching carefully.

Immediately pour into ungreased 13x9 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.