Preheat oven to 350°F (175°C).
Grease a 9x13-inch baking dish with butter.
Arrange slices of bread in the baking dish, slightly overlapping.
Layer the sliced apples over the bread.
In a mixing bowl, whisk together eggs, milk, brown sugar, cinnamon, and vanilla extract.
Pour the egg mixture evenly over the bread and apples.
Cover the dish with plastic wrap or foil and refrigerate overnight to allow the bread to soak in the custard.
The next morning, remove the dish from the fridge and let it rest for 10 minutes.
Bake for 45-50 minutes, or until golden and puffed.
Let it cool for about 10 minutes before serving.
Serve warm, topped with powdered sugar or maple syrup if desired.