In a large bowl, whisk the eggs, milk, vanilla, sugar and cinnamon until blended. Arrange the bread in a large shallow baking dish, overlapping the slices if necessary.
Pour the milk mixture over the bread and let stand for at least 30 minutes, carefully turning the bread with a wide spatula and rearranging it so it is evenly moistened halfway through the standing time.
Position one oven rack in the upper third of the oven and one in the lower third and preheat the oven to 400°. Put 2 tablespoons of the butter on each of two baking sheets with sides and place in the oven until the butter melts and pans are hot, about 5 minutes. Tilt the pans so the butter covers them evenly. Remove from the oven.
Place 5 to 6 slices of the soaked bread on each of the hot pans, spacing them evenly. Bake for 15 minutes. Take the pans out of the oven and turn each piece of bread over with a wide spatula. Return the pans to the oven, placing the one from the top rack on the bottom rack on the top, and bake until the bread is puffed and evenly browned, 15 to 20 minutes more.
Transfer to a platter, dust with powdered sugar and serve with maple syrup, if desired.