Peel and slice the banana in half lengthways and set aside. Crack the eggs into a shallow bowl, add the vanilla and cinnamon then whisk together with a fork.
Soak the bread slices in the egg mixture for a couple of minutes, turning halfway.
Place a large dry frying pan on a medium heat and toast the hazelnuts until golden, then tip into a pestle and mortar and bash until fine.
Return the pan to the heat with the butter (or 1 teaspoon of oil). Once hot, carefully place one egg-soaked slice into the pan and break over the chocolate. Place the remaining slice on top and sandwich together.
Add the banana slices to the pan and cook, without moving, for 2 minutes, until lightly golden.
Use a spatula to carefully flip the bread and banana slices and cook for a further 2 minutes, until light golden on both sides.
Lightly dust the top of the sandwich with the icing sugar and flip back over for a few seconds to caramelise.
Carefully slide the sandwich onto a board (the caramel will be very hot!), cut in half diagonally, then transfer to a plate.
To serve, place half a sandwich on each plate, top with 2 pieces of caramelised banana and a dollop of yoghurt, and finish with a good dusting of crushed hazelnuts.