Preheat the oven to 350 degrees F (175 degrees C). Lightly butter an 8x8-inch baking dish.

Pour bread cubes into the prepared dish.

Beat milk, eggs, sugar, and vanilla together in a large bowl until evenly mixed; pour mixture over bread cubes. Arrange butter chunks atop bread mixture. Set aside for bread to absorb liquid, about 10 minutes. Sprinkle the top with cinnamon.

Bake in the preheated oven until top is golden, 45 to 50 minutes.