Prepare a 9x13 baking dish by lightly spraying it with nonstick cooking spray or coating lightly in butter. I didn't do this and some of the cooked bread got stuck.
Cut a stale baguette into slices and then cut each slice into one-inch cubes.
Place the cut bread into the baking dish.
Pour the milk into a medium bowl.
Add the four eggs to the milk and whisk it all together until slightly foamy.
Add the sugar and stir until combined.
Add the cinnamon and mix.
Add the vanilla and mix.
Sprinkle the blueberries evenly over the bread.
Pour the custard mix evenly over the surface of the bread.
Press the bread down into the blueberries. Be careful to do it gently so you don't break the berries. I actually tossed the bread in the liquid by using my hands.
Cover the dish with aluminum foil and put into the fridge. Leaving it overnight works best.
Take the dish out of the fridge the next morning and let it sit on a counter while you preheat the oven.
Preheat the oven to 350 degrees.
Cut the butter into cubes and dot it over the surface of the bread.
Bake the French toast with the foil on for about 30 minutes.
Remove the foil and let the French toast bake for another 15 minutes until the bread is golden and the blueberries bubbling.
Serve plain or with maple syrup or a sprinkle of confectioner's sugar. I made cinnamon sugar with 1/4 cup of sugar and 1 tsp of cinnamon and we used that.