In a medium bowl, add bananas and mash well. Stir in non-dairy milk, cacao powder and chia seeds, mixing well.
Let set for about 30 minutes, and give a good stir. Repeat one more time, stirring again after 30 minutes. This step is an important step, as the seeds need to be stirred once or twice before completely gelling up and setting. If not stirred, the pudding mixture will be soupy.
Cover and place in the refrigerator for at least 6 hours, or overnight.