Generously butter a 9 x 13-inch baking dish or spray it with non-stick cooking spray.

Spread half the bread cubes into the bottom of the baking dish. Cut the cream cheese into small pieces, and dot the pieces on top of the bread cubes. Pour the blueberries evenly over the bread and cream cheese. Add the remaining bread pieces in an even layer. Set aside.

In a large bowl, whisk the 12 eggs. Add the heavy cream, milk, sugar, and vanilla. Pour the egg mixture over the bread in the pan. Make sure to pour it slowly and evenly so all pieces are covered and it doesn’t spill out. Cover the pan with plastic wrap and refrigerate at least 8 hours or up to overnight.

The following morning, remove the casserole from the fridge and take off the plastic wrap. Let the casserole sit out at room temperature for 30 minutes before baking, and preheat the oven to 350 degrees.

Cover the casserole with foil and bake for 30 minutes. After 30 minutes, uncover and continue to bake an additional 25 to 30 minutes until the top is golden brown and the center is set. While the casserole bakes and cools, make the blueberry sauce.

In a medium saucepan, combine the water, sugar, and cornstarch over low-medium heat. Whisk until the ingredients are combined and continue to whisk every few minutes. The mixture will begin to simmer and then boil. Stir constantly once the mixture starts to simmer, because it will thicken quickly.

After it starts to boil, allow it to boil for 1 to 2 minutes then add the blueberries. Stir the blueberries for an additional 3 to 4 minutes, then turn off the heat and add the butter. Stir until the butter melts completely.

Pour the blueberry sauce over the warm casserole or serve it with individual pieces. The casserole stays fresh for about 4 to 5 days when stored in the refrigerator.