1. Preheat oven to 356 degrees F. In a medium bowl, mix flour, baking powder and salt.
2. In a large bowl, with mixer at medium speed, beat brown sugar, peanut butter, and margarine or butter until smooth, about 2 min. Reduce speed to low. Beat in eggs and vanilla. Beat until smooth. Beat in flour mixture just until combined (dough will be stiff).
3. Place 1/3 of dough (about 1 3/4 C) in another large bowl. Stir in melted chocolate until blended. Fold in 3/4 C semi-sweet chocolate pieces.
4. Pat 1/3 of remaining peanut butter dough into ungreased 13x9 inch pan. In random pattern, drop chocolate dough and remaining peanut butter dough on top of peanut butter layer. Pat down with hands. Sprinkle with remaining chocolate pieces.
5. Bake 25-30 min. until toothpick inserted in center comes out clean. Cool on rack.
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