Butter 15 x 10 x 1 inch jelly-roll pan. Set aside. Combine butter, sugar, water and salt in heavy 2 quart saucepan. Cook over medium heat, stirring constantly, until mixture comes to a full boil.

Continue cooking, stirring occasionally, until candy thermometer reaches 300 or small amount of mixture dropped into ice water forms a hard brittle strand [18-22 minutes]. Remove from heat; stir in vanilla.

Pour mixture onto prepared pan; spread to desired thickness. Sprinkle chocolate chips over hot candy; let stand 5 minutes. Spread melted chocolate evenly over candy. Cool completely; break into pieces.