Preheat the oven to 375°F. Grease a 9x13-inch baking pan with butter.
In a large bowl, whisk together the pumpkin puree, milk, eggs, sugar, pumpkin pie spice and vanilla extract.
Add the cubed bread to the bowl and stir until it is well coated then pour the bread mixture and any liquids into the prepared baking pan.
Cover the pan with foil and bake the French toast for 30 minutes until the egg mixture is almost fully set then remove the foil and continue baking the French toast for 20 to 25 minutes until the bread is deep golden brown.
Remove the French toast from the oven, spoon it onto serving plates and top it with maple syrup.