In a small bowl, beat the peanut butter, brown sugar and 2 tablespoons flour until blended; set aside.
In a microwave-safe bowl, melt chocolate and butter; stir until smooth; cool slightly. In a large bowl, beat the eggs, sugar and water; stir in chocolate mixture and vanilla. Combine baking powder and remaining flour; gradually add to chocolate mixture.
Fill paper-lined muffin cups one-fourth full. Top each with about 1 teaspoon of peanut butter mixture. Spoon remaining batter over the top. Bake at 350° for 13-15 minutes or until centers are set (do not overbake). Cool for 5 minutes before removing from pans to wire racks.