Line the bottom and sides of a large, rimmed baking sheet with parchment paper. Coat paper with nonstick cooking spray.

In a medium-sized, heavy-bottomed saucepan, combine butter, sugar, water and salt. Bring to a boil over high heat, stirring often. Reduce heat to medium; boil gently without stirring until temperature reaches 305 to 310 F (hard crack stage).

Remove from heat; immediately pour onto prepared pan. Mixture will spread to about 10 x 7 inch rectangle. Let stand at room temperature until cooled completely.

Spread melted chocolate evenly over toffee; sprinkle rosemary and salt evenly over chocolate. Refrigerate until chocolate is set, about 30 minutes. Cut or break toffee into pieces. May be stored in refrigerator or at room temperature in an airtight container for up to 1 week.