Cut French bread into 1-inch cubes. Place them in greased baking dish.
Spread cubes evenly across the bottom. Don't pack them too tightly.
Whisk eggs in large bowl. Add milk and heavy cream.
Mix in sugar and vanilla extract. Add cinnamon and salt together. Whisk until completely smooth.
Pour custard over bread cubes slowly. Press bread down gently with spatula.
Make sure all pieces get coated. Drizzle melted butter on top.
Cover dish with plastic wrap tightly. Put in refrigerator for 8 hours.
This step is very important. The bread soaks up custard perfectly.
Preheat oven to 350°F next morning. Remove plastic wrap from dish carefully.
Bake for 45-50 minutes until golden. The center should be set completely.
Let casserole cool for 5 minutes. Dust with powdered sugar generously.
Cut into squares and serve warm. Add maple syrup if desired.