Start by cutting your challah or brioche loaf into roughly 1-inch cubes. Spread the cubes out on a baking sheet and let them sit while you prepare the custard mixture.
In a large bowl, whisk together 4 large eggs, 2 cups of milk, 1/2 cup granulated sugar, 1 tablespoon cinnamon, and 1 teaspoon vanilla extract until combined and smooth.
Add the cubed bread and 1/2 cup raisins to the custard mixture. Gently fold everything together, making sure all the bread cubes are coated with the custard.
Butter a 9x13-inch baking dish generously to prevent sticking. Pour the bread and custard mixture into the dish, spreading it out evenly.
Cover the dish with plastic wrap and refrigerate for at least 2 hours or overnight to allow the bread to soak up the custard.
Preheat your oven to 350°F (175°C). Remove the baking dish from the fridge and bake uncovered for 45-50 minutes, or until the top is golden and the custard is set.
Once baked, drizzle with 1/4 cup pure maple syrup and sprinkle powdered sugar over the top if desired. Serve warm.