Heat the milk up for about 30-60 seconds (just enough for it to be warm and melt the chocolate when poured over).
Pour over the chocolate, and stir until the chocolate is melted.
Add the cacao powder.
Add the banana and the chocolate-cacao-milk mixture to a small blender, and blend until well incorporated (45-60 seconds).
Divide the liquid up between two glass jars.
Add 4 tbsp of chia seeds to each jar, and stir them in. Let the jars sit on the counter for about 20-30 minutes, then give them another stir (to prevent any lumps from forming) and refrigerate for 6-8 hours (or overnight).
Once the chia pudding has been refrigerated, you can garnish it with some banana slices, a piece of dark chocolate, and honey or agave (optionally) for a finishing touch.