Slice the challah or brioche bread into thick pieces (about 1 inch).

In a large bowl, whisk together eggs, milk, granulated sugar, vanilla extract, and cinnamon until smooth.

Layer half of the sliced bread in a greased baking dish and pour half of the egg mixture over it. Repeat with remaining bread and egg mixture.

Cover the dish with plastic wrap and refrigerate overnight.

Preheat the oven to 350°F (175°C). Remove the casserole from the fridge while the oven heats.

Bake uncovered for 45-50 minutes until puffed and golden brown. Serve warm with powdered sugar and maple syrup.