Add the dark chocolate to a jug, pour in the milk and the espresso. Microwave until the mixture is warm and the chocolate has melted (you can do this in a pan if you prefer).
Place the oats, banana, hazelnuts, chia seeds, maca powder, the water and approx. a fifth of the milk mixture to your smoothie maker or blender. Add in an extra splash of cold water if you think the liquid is too warm (see warning above about hot liquids and smoothie makers).
Blend on high for a minute until the oats and chia seeds have been completely incorporated.
Whilst it's blending, further heat the rest of the milk mixture until hot, but not boiling.
Pour the blended oat mix into your cup, stir in the milk/chocolate mix and serve.