Preheat your oven to 350°F (175°C) and generously grease a 9x13 inch baking dish with butter or cooking spray.

Cut your day-old challah or brioche into bite-sized cubes and layer them evenly in the greased baking dish. Scatter fresh blueberries generously over the top.

In a large mixing bowl, whisk together the eggs, milk, heavy cream, maple syrup, vanilla extract, ground cinnamon, and a pinch of salt until fully blended.

Pour the custard mixture evenly over the bread cubes and blueberries, ensuring every piece is well coated. Let this mixture sit for 30 minutes at room temperature or refrigerate overnight.

Carefully place the soaked casserole in the preheated oven and bake for about 45-50 minutes, until the top is golden brown and the custard is set.

Once baked, let the casserole cool for about 10 minutes before slicing into squares. Serve warm, topped with maple syrup, powdered sugar, and whipped cream.