In large microwavable bowl, microwave chocolate chips and butter uncovered on Medium (50%) 1 minute 30 seconds, stirring after 30 seconds, and then at 15-second intervals; stir until smooth. Stir in 1 1/3 cups of the sugar with whisk. Add whole eggs, one at a time, stirring just until blended after each addition. Gradually add flour, baking powder and salt, stirring with whisk just until blended. Stir in 2 teaspoons of the vanilla. Spread batter in pan.