Preheat the oven to 350°F (175°C). Grease a 9 x 13 inch baking dish with butter.

In a large bowl, whisk the eggs until combined. Add milk, heavy cream, sugar, vanilla, and cinnamon. Whisk until smooth.

Spread half of the cubed French bread in the prepared dish so the pieces form an even layer.

Scatter half of the blueberries over that bread layer.

Add the remaining bread and top with the rest of the blueberries.

Pour the egg mixture evenly over the bread and berries. Press lightly so the bread soaks up the liquid and the custard reaches the bottom pieces.

Let the casserole sit at room temperature for 20 to 30 minutes to absorb the custard. For best texture and depth of flavor, cover and refrigerate overnight.

Bake in the preheated oven for 45 to 50 minutes, or until the custard is set in the center and the top is golden and slightly puffed. A knife inserted near the center should come out mostly clean.

Allow the casserole to cool slightly before serving. Dust with powdered sugar if you like, then slice and serve warm.