Set out a deep oven-proof dish (approx. 30x20cm) that fits the 6 brioche rolls in 2 neat rows of 3. Lightly grease with butter and set aside.
Place the brioche rolls onto a board, spread each one with 1 tbsp nutella, then sandwich together to make 6 brioche sandwiches. Arrange in the prepared dish.
In a large mixing bowl, whisk together the eggs and milk and set aside.
In a medium saucepan, add the water and bring to a simmer. Add the sugar and let it bubble away gently, without stirring, for about 3-5 minutes - or until it turns a lovely caramel colour.
Reduce the heat and carefully pour in the cream, whisking all the time - it will splutter but keep whisking. Stir in 1/2 tsp of flakey salt (use just a good pinch if using fine salt). Remove from the heat and leave to cool slightly (you don’t want it to be too hot or it will scramble the eggs when you add them). It only needs to cool for 10 minutes or so.
Once cooled slightly, pour the caramel into the beaten eggs and whisk to combine.
Pour over the brioche-nutella sandwiches and leave it to settle for 10 minutes, pushing the brioche down every now and then to ensure they are covered in the caramel custard as much as possible.
Cover with parchment paper and in that 10 minutes, press down the brioche down every now and then to ensure they are covered in the caramel custard as much as possible.
Meanwhile, preheat the oven to 160°C Fan.
Bake for 25-30 minutes - or until golden and the custard has set. Remove from the oven and sprinkle with the chopped chocolate straight away (you need to be quick so the heat from the pudding melts the chocolate). Leave to sit for 5 minutes.
Sprinkle with raspberries and decorate with sprigs of fresh mint.
Serve with scoops of vanilla ice cream, if desired.