In a medium bowl, whisk together the melted butter, brown sugar, eggs, milk, vanilla, and 1 tsp of cinnamon until fully combined and smooth.
Add the cubed French bread to a buttered or cooking-sprayed 9x13 pan. Pour the custard mixture over the top and toss gently until every piece of bread is fully coated and soaking up the custard.
Sprinkle the top evenly with the 1 Tbsp of brown sugar and remaining 1 tsp of cinnamon. This creates the golden, lightly caramelized crust on top that makes this casserole so visually beautiful when it comes out of the oven.
Bake for 25–30 minutes until the top is golden brown and the custard is set through the center.
Dust generously with powdered sugar and serve immediately.