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.