Preheat your oven to 350°F (175°C). Butter a 9x13-inch baking dish generously and set aside.
Spread the brioche cubes on a large baking sheet and let them dry out at room temperature for 2-3 hours, or toast them in a 300°F oven for 8-10 minutes until slightly dried but not browned.
In a medium saucepan over medium heat, warm the milk and cream until steaming but not boiling, about 5 minutes. Remove from heat.
In a large mixing bowl, whisk together the eggs, granulated sugar, vanilla extract, cinnamon, and salt until smooth and well combined.
Slowly pour the warm milk mixture into the egg mixture while whisking constantly to temper the eggs. This prevents them from scrambling.
Place the dried brioche cubes in the prepared baking dish and pour the custard mixture evenly over the top. Press down gently with a spatula to ensure all pieces are submerged.
Let the mixture sit for 30-45 minutes at room temperature, pressing down occasionally so the brioche absorbs the custard fully.
Drizzle the melted butter over the top and sprinkle with turbinado sugar for a caramelized finish.
Bake for 45-55 minutes until the top is golden brown and the center has just a slight jiggle when gently shaken. The internal temperature should reach 170-175°F.
Let the bread pudding cool for 15 minutes before serving warm with your favorite toppings.
