Combine sugar, butter, marshmallows, milk, and salt in a large heavy 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, stirring occasionally, 5 minutes or until mixture reaches soft ball stage (236°).
Remove from heat; add chocolate, stirring until melted. Stir in pecans and vanilla. Spread in a buttered 8-inch square baking dish. Cool completely; cut into 1 1/2-inch squares.