Lightly grease a deep ovenproof baking dish (approximately 30 x 20 cm) with butter.
Spread 1 tablespoon of Nutella onto each sliced brioche roll and sandwich together.
Arrange the brioche sandwiches neatly inside the prepared baking dish.
In a large mixing bowl, whisk together the eggs and milk until fully combined.
Pour the water into a medium saucepan and bring to a gentle simmer.
Add the caster sugar and allow it to bubble gently without stirring for 3–5 minutes, or until it develops a rich golden caramel color.
Reduce the heat and carefully pour in the double cream while whisking continuously. The mixture will bubble vigorously.
Stir in the flaky salt and remove from the heat.
Allow the caramel to cool for approximately 10 minutes.
Slowly whisk the cooled caramel mixture into the egg and milk mixture until smooth.
Pour the salted caramel custard evenly over the brioche sandwiches.
Let the pudding soak for 10 minutes, gently pressing the brioche down occasionally to help absorb the custard.
Cover loosely with parchment paper.
Preheat the oven to 160°C Fan (320°F).
Bake the Salted Caramel Bread and Butter Brioche Pudding for 25–30 minutes, or until golden brown and the custard is set.
Remove from the oven and immediately sprinkle over the chopped dark chocolate.
Allow the pudding to rest for 5 minutes so the chocolate softens and melts slightly.
Garnish with fresh raspberries and mint sprigs.
Serve warm with generous scoops of vanilla ice cream.