Spray a 9x13-inch baking pan with non-stick cooking spray.
Melt the butter and SunButter together in a large Dutch oven or stock pot over medium heat. Add the marshmallows and melt, stirring occasionally. Once everything has melted together and is fully combined, take off the heat and add the popcorn and chocolate chips. Stir well to combine everything.
Spread the mixture into the prepared pan and press down firmly. Let cool for at least 30 minutes at room temperature (or store in the refrigerator) before cutting into squares. Store leftovers in an airtight container in the refrigerator for up to 2 days.