Combine sugar, milk, and chocolate syrup in a 3-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, stirring occasionally, until mixture reaches soft ball stage (236°).
Remove from heat; add butter, vanilla, and marshmallow creme (do not stir). Cool to lukewarm (110°). Beat with a wooden spoon until mixture thickens. Pour into a buttered 13- x 9- x 2-inch baking pan. Cool and cut into 1-inch squares.