Preheat the oven to 160°C. Grease a 24cm x 12cm loaf pan. Line the base and long sides of the pan with 2 layers of baking paper, leaving 5cm overhanging (this will make it easier to remove the pudding to serve).
Halve croissants horizontally and spread with chocolate hazelnut spread. Reserve 2 croissants and cut the remaining croissants crossways into 6cm pieces. Press into prepared pan. Top with whole halved croissants, tucking the edges in at the sides.
Whisk eggs, milk, vanilla and caster sugar in a bowl. Pour egg mixture over croissants and stand for 1 hour to soak. Bake, covering with foil halfway if top croissants get too dark, for 1 hour or until a skewer inserted in pudding comes out clean and top is golden.
Place maple syrup in a saucepan and bring to the boil. Place the pudding on a tray lined with baking paper and carefully pour over the hot syrup to coat the top. Stand for 1 hour to cool. Pull pudding from tin and place on a serving platter. Drizzle with chocolate, scatter over hazelnuts and slice thickly to serve.