Combine sugar, chocolate, milk, syrup, butter, and salt in a 10-inch cast-iron skillet. Bring to a boil, stirring constantly. Boil 2 minutes or until mixture reaches soft ball stage (236°).
Remove from heat; add pecans and vanilla (do not stir). Cool to lukewarm (110°). Beat with a wooden spoon until mixture is thick and creamy. Pour into a buttered 8-inch square baking pan. Cool completely before cutting into 1-inch squares.