Brown butter lightly in a 3- quart saucepan. Add sugar, milk, syrup, and salt; stir well. Cook over medium heat, stirring constantly, until sugar dissolves. Stir in chocolate. Cover and continue to cook 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, stirring occasionally, until mixture reaches soft ball stage (240°).
Remove from heat. Cool to lukewarm (110°). Add vanilla; beat with a wooden spoon until mixture thickens and begins to cool. Pour into a buttered 9-inch square baking pan. Cool and cut into 1 1/2-inch squares.
Note: 1/2 cup peanut butter may be stirred into chocolate mixture just before removing from heat.