Grease a 9×13 inch baking dish well with butter and set it aside.

Cut the bread into 1 to 1.5-inch cubes. Spread half of the cubes evenly in the prepared dish. Scatter half of the blueberries over that layer.

In a medium bowl, whisk together the eggs, milk, heavy cream, sugar, vanilla, and cinnamon until smooth and homogenous.

Pour half of the custard over the first bread layer. Gently press down so the custard soaks into the cubes.

Add the remaining bread cubes and the rest of the blueberries on top. Pour the remaining custard over everything and press again so the bread is submerged and moist.

Cover the dish tightly with plastic wrap or foil and refrigerate overnight, or for at least 6 hours.

In the morning, remove the wrap and preheat the oven to 350°F (175°C).

Bake the casserole for 45 to 55 minutes, until it is puffed, golden on top, and the center is set (an instant-read thermometer should read about 160°F to 165°F).

Let it cool for 5 to 10 minutes before slicing and serving.