Beat sugars and butter at medium speed of a mixer until well-blended (about 4 minutes). Add egg whites and vanilla; beat well. Lightly spoon the flour into dry measuring cups, and level with a knife. Combine flour, baking powder, and salt; stir well with a whisk. Add flour mixture to sugar mixture; beat at low speed just until blended.
Spread batter evenly into an 8-inch square baking pan coated with cooking spray; sprinkle evenly with morsels. Bake at 350° for 28 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack.