1. Preheat oven to 350°. Bake pecans in a single layer in a shallow pan 8 to 10 minutes or until toasted and fragrant, stirring halfway through. Cool completely (about 15 minutes).
2. Meanwhile, bring brown sugar and next 3 ingredients to a boil in a heavy Dutch oven over medium heat, stirring constantly. Boil, stirring occasionally, 6 to 8 minutes or until a candy thermometer registers 236° (soft ball stage). Remove sugar mixture from heat. Add chocolate baking squares to sugar mixture. (Do not stir.)
3. Let sugar mixture stand until candy thermometer reaches 150° (20 to 25 minutes). Stir in vanilla and pecans using a wooden spoon; stir constantly 1 to 2 minutes or just until mixture begins to lose its gloss. Quickly drop by heaping tablespoonfuls onto wax paper; let stand until firm (10 to 15 minutes).