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

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

Spread half of the cubed French bread evenly in the prepared dish. Scatter half of the blueberries over the bread. Add the remaining bread, then top with the rest of the blueberries.

Pour the egg mixture evenly over the layered bread and berries. Press the bread gently so it soaks up the liquid.

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

Bake for 45 to 50 minutes, or until the custard is set in the center and the top is golden brown.

Allow it to cool slightly before serving. Dust with powdered sugar if desired, then slice and serve warm.