In a large mixing bowl, whisk together the eggs, heavy cream, whole milk, granulated sugar, vanilla extract, ground cinnamon, and a pinch of salt until well blended.
Dip each slice of challah bread in the egg mixture, allowing it to soak for about 30 seconds on each side to absorb the flavors.
Preheat your oven to 375°F (190°C). Grease a large baking dish with butter or non-stick spray.
Arrange the soaked bread slices in the baking dish, overlapping them slightly if necessary.
Pour any remaining egg mixture over the bread and cover the baking dish with aluminum foil. Let it sit for about 15 minutes for the bread to absorb more of the custard.
Remove the foil and sprinkle the brown sugar evenly over the top of the bread.
Bake in the preheated oven for 25 minutes, then remove from the oven and broil on high (watch carefully!) for an additional 2-3 minutes until the sugar caramelizes and creates a crispy top.
Allow to cool for a few minutes before serving; cut into squares for presentation.
Serve warm, topped with fresh berries and mint leaves for garnish, if desired.