Using a hand-held or stand mixer fitted with a paddle, beat the melted butter, vanilla, peanut butter, and confectioners’ sugar until smooth. Drop big spoonfuls of most of the chocolate batter randomly into the prepared pan. Top with random spoonfuls of all of the peanut butter batter. Drop random spoonfuls of remaining chocolate batter over any bare spots or heavy peanut butter spots. Using a knife, swirl the batters together.