In a small bowl, whisk together 1/2 cup of almond milk and 1/3 cup arrowroot starch. Set aside.
In a blender, combine the remaining 2 1/2 cups of almond milk along with the banana, maple syrup, cocoa powder, and salt.
Transfer the blender mixture to a medium saucepan. Cover the pan and heat the mixture over medium heat. When the mixture begins to bubble, remove the cover and slowly add the arrowroot mixture to the pan and whisk. Heat for an additional 3 minutes (uncovered), whisking constantly, until slightly thickened. The mixture should stick to the back of a wooden spoon.
Transfer the pudding to a glass container (or similar). Let it cool for a few minutes, then cover and refrigerate for several hours to thicken.