Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Divide the chocolate between the glasses, spread around the rims of the glasses with a spoon, leave to dry.
Chop the bananas into pieces, puree the bananas in a blender along with all the other ingredients up to and including the oat milk. Pour the shake into the glasses, serve immediately.