Since the ice cream mixture is cold, it will harden the peanut butter if not swirled after each addition. I’ve tried swirling the peanut butter after filling up the cartons in alternating layers of the ice cream mixture and peanut butter. It had hardened too much, stuck to the chopstick when pulling the chopstick back out, and was not able to be swirled. After freezing, the ice cream had peanut butter chunks instead of swirls. The best way to get the swirls evenly distributed in the ice cream is to swirl the peanut butter immediately after each addition.