Combine sugar, syrup, water, and salt in a 2-quart saucepan. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches soft crack stage (280°).
Remove from heat; add chocolate and pecans. Beat constantly with a wooden spoon until mixture begins to thicken. Working rapidly, spread mixture evenly in a thin layer on a buttered baking sheet. Cool completely; break into pieces. Store candy in an airtight container, or wrap in plastic wrap.