Preheat your oven to 425°F (220°C). Generously butter a 13x9 inch baking pan to prevent sticking, then set the pan aside until ready to assemble the casserole.

Melt the butter in a small saucepan over medium heat. Once melted, stir in 1 cup of packed brown sugar until dissolved and the mixture is smooth. Pour this brown sugar butter mixture into the bottom of the prepared 13x9 inch pan and spread it out evenly.

Arrange the cubed French bread evenly over the layer of brown sugar butter in the pan, making sure the cubes are distributed across the surface.

In a medium bowl, whisk together the eggs, milk, vanilla extract, and 1 teaspoon of cinnamon. Pour this egg mixture evenly over the bread cubes, ensuring all pieces are saturated. I always gently press the bread cubes down with a spatula to help them soak up the custard.

Sprinkle the bread evenly with the remaining 1 tablespoon of brown sugar and 1 teaspoon of cinnamon. Bake in the preheated oven for 25-30 minutes, or until the top is golden brown. Remove from the oven and, if desired, dust with powdered sugar before serving.