1. Combine first 3 ingredients in a large mixing bowl; set aside.
2. Combine sugar and evaporated milk in a buttered Dutch oven.
3. Cook sugar mixture over medium heat, stirring constantly, until mixture reaches soft ball stage or candy thermometer registers 234°; pour over chocolate mixture. Beat at high speed with an electric mixer or with a wooden spoon until mixture thickens and begins to lose its gloss. Stir in vanilla and chopped pecans.
4. Spread into a buttered 15- x 10-inch jellyroll pan. Cover and chill until firm. Cut into 1-inch squares. Store in an airtight container at room temperature.