Preheat the oven to 175°C and grease a baking dish with butter.
Pour the milk and cream into a saucepan. Split the vanilla pods and place them in the saucepan. If using vanilla essence add this to the milk and cream. Bring to the boil, take off the heat and allow to cool.
In a bowl whisk the egg yolks and sugar, then using a sieve strain the cream and milk into the eggs and sugar, stirring continuously.
Butter the slices of bread and cut into four triangles.
In the baking dish, starting at one end, place three triangles of bread, then sprinkle with some banana, walnuts and chocolate drops. Repeat this process until all of the bread, bananas, walnuts and chocolate chips are used up. Carefully pour over the custard. Leave to stand for 20 minutes pushing down on the bread occasionally. The idea is for the bread to soak up the custard.
Place the baking dish in a larger baking tray half filled with boiling water and place in the oven for 30 minutes until cooked but with a slight wobble. Do not allow the custard to overcook otherwise it will scramble.
Leave to rest for ten minutes. Dust with icing sugar before serving.