Preheat oven to 350 degrees F. In medium bowl, mix flour, baking powder, and salt.
In large bowl, with mixer at medium speed, beat brown sugar, peanut butter, and margarine or butter until smooth, about 2 minutes. Reduce speed to low. Beat in eggs and vanilla; beat until smooth. Beat in flour mixture just until combined (dough will be stiff).
Place one-third of dough (about 1 3/4 cups) in another large bowl. Stir in melted chocolate until blended; fold in 3/4 cup semisweet-chocolate pieces.
Pat half of remaining peanut-butter dough into ungreased 13" by 9" metal baking pan. In random pattern, drop chocolate dough and remaining peanut-butter dough on top of peanut-butter layer; pat down with hand. Sprinkle with remaining chocolate pieces.
Bake 25 to 30 minutes until toothpick inserted in center comes out clean. Cool in pan on rack.