To make the ice cream: Combine the plant milk, bananas, chocolate syrup, and cocoa powder in the blender. Blend until smooth. Pour into your ice cream machine and churn according to the manufacturer's instructions.
To make the peanut butter swirl: When the ice cream has about 10 minutes left to churn, place the peanut butter in a medium size glass bowl. Heat in the microwave for about 60 seconds, until liquid. Stir and add the powdered sugar. Stir or whisk again until smooth.
Scoop the peanut butter mixture into a plastic freezer bag and seal closed, while removing any extra air.
To combine the peanut butter swirl with the ice cream: Scoop the finished ice cream into a freezer safe container. Use a pair of scissors to snip a corner off the bag with the peanut butter. Squeeze the peanut butter over the ice cream and then swirl with a knife. The peanut butter will harden very quickly and form bite-size chunks. (It shouldn't be liquid at all.)
Place the airtight container in the freezer for at least a couple hours to firm up completely before serving. Enjoy!