Line a 13-in. x 9-in. baking pan with foil; grease the foil with 1-1/2 teaspoons butter. Sprinkle 1 cup pecans into prepared pan; set aside.
In a small heavy saucepan, combine the brown sugar, water and the remaining butter. Bring to a boil over medium heat, stirring constantly. Cook without stirring until a candy thermometer reads 290° (soft-crack stage). Remove from the heat; stir in vanilla and baking soda. Pour into prepared pan (do not scrape sides of saucepan).
Sprinkle with chocolate; spread with a knife when melted. Sprinkle with the remaining pecans. Let stand until set, about 1 hour. Break into pieces.