HEAT oven to 350 degrees F. Spray an 8 x 8-inch baking pan with no-stick cooking spray.
COMBINE flour, baking powder and salt in small bowl.
COMBINE brown sugar, peanut butter and butter in bowl of electric mixer. Beat until light and creamy. Add eggs and vanilla. Beat until fluffy. Stir in flour mixture just until blended. Spread in prepared pan.
DRIZZLE melted chocolate over batter. Using a small, sharp knife, swirl the chocolate into the top of the batter to create a marbled effect.
BAKE 30 minutes or until toothpick inserted in center comes out clean. Cool in pan on cooling rack. Cut into 24 bars.