Preheat your oven to 350°F (175°C).

Grease a 9x13-inch baking dish generously with unsalted butter.

Tear the day-old bread into bite-sized chunks and spread them across the bottom of the baking dish.

Whisk together eggs, half-and-half, light brown sugar, vanilla extract, ground cinnamon, and kosher salt until smooth.

Pour the custard mixture over the torn bread, pressing down gently to ensure it's well-coated.

Cover the dish with plastic wrap and refrigerate overnight.

If baking immediately, let the casserole rest at room temperature for about 1 hour.

Mix topping ingredients until crumbly, then chill until ready to use.

Sprinkle the topping over the soaked bread and bake for 45-50 minutes until golden brown.

Allow to cool for 5-10 minutes before serving, topping with maple syrup or powdered sugar.