Line a 6-qt. oval slow cooker with heavy foil. In a large bowl combine melted butter and both sugars. Stir in egg and vanilla. Stir in flour, baking soda, and salt. Stir in half of the chocolate pieces. Spread mixture in prepared cooker; top with remaining chocolate pieces.
Cover and cook on high 2 hours or until surface is evenly brown, giving crockery liner a half-turn once halfway through, if possible.
Turn off cooker. If possible, remove crockery liner from cooker. Cool, uncovered, at least 1 hour. Using foil, lift out cookies. Cut into bars.