Combine sugar, milk, chocolate, syrup, and salt in a Dutch oven. Cook over low heat, stirring constantly, until sugar is dissolved.
Continue to cook, stirring constantly, until mixture reaches soft ball stage (240°). Remove from heat; add butter and vanilla (do not stir). Cool to lukewarm (110°).
Add pecans; beat with a wooden spoon 2 to 3 minutes or until mixture is thick and begins to lose its gloss. Pour into a buttered 8-inch square pan. Mark warm fudge in 1 1/3- inch squares. Cool and cut.