Heat oven to 375°F. Spray 13x9-inch pan with cooking spray.
In large microwavable bowl, microwave butter and 2 cups of the chocolate chips uncovered on High 1 minute; stir. Microwave 30 seconds to 1 minute longer until melted and mixture can be stirred smooth. Cool slightly. Stir in eggs and gingerroot.
In medium bowl, mix flour, granulated sugar, brown sugar and baking powder with wire whisk. Add flour mixture to chocolate mixture, stirring just until blended. Stir in remaining 2 cups chocolate chips. Spread batter in pan.
Bake 30 to 33 minutes or until edges are set and toothpick inserted in center comes out almost clean. Cool completely in pan on cooling rack, about 2 hours. Cut into 6 rows by 4 rows.