Prepare the Bread: Lay the brioche slices in a single layer in a greased 9x13-inch baking dish.
Make the Custard: In a large bowl, whisk together the eggs, heavy cream, milk, granulated sugar, vanilla extract, and salt until everything is well combined.
Combine: Pour the custard mixture evenly over the brioche slices. Press the bread gently to ensure it's soaked through.
Chill: Cover the dish with plastic wrap and refrigerate overnight (or at least for 4 hours) to allow the flavors to meld.
Bake: Preheat your oven to 350°F (175°C). Remove the dish from the fridge; sprinkle a layer of brown sugar on top. Bake for about 45-50 minutes, or until puffed and golden.
Cool & Serve: Allow to cool for a few minutes, then serve warm with your favorite toppings.