Cut French bread into 1-inch cubes and place evenly in a large baking dish.
In a mixing bowl, whisk together eggs, milk, heavy cream, sugar, maple syrup, and 2 teaspoons vanilla extract until well combined.
Pour the custard mixture over the bread cubes, pressing gently to soak all pieces.
Cover the baking dish with plastic wrap and refrigerate for at least 2 hours or overnight.
In a separate bowl, combine cold butter, brown sugar, all-purpose flour, 2 teaspoons vanilla extract, and salt. Mix with fingers or pastry cutter until crumbly.
Preheat the oven to 350°F (175°C).
Remove casserole from fridge, sprinkle crumb topping evenly over bread mixture, and bake for 45-50 minutes until golden brown and set.
Let casserole sit a few minutes after baking. Serve warm, optionally drizzled with maple syrup or topped with fresh fruit or whipped cream.