In a standing electric mixer fitted with the paddle, beat 2 sticks of the butter with the granulated sugar at medium speed until very fluffy, 5 minutes. Beat in the eggs one at a time, then add the vanilla. In 3 alternating batches, beat in the flour mixture and the milk. Transfer one-fourth of the batter to a medium bowl and stir in the melted chocolate. Beat the peanut butter into the remaining batter.