Preheat your oven to 425 degrees Fahrenheit. While the oven heats up, butter a 13x9 inch baking pan and set it aside.
In a small saucepan, melt the butter over medium heat. Stir in the brown sugar until dissolved and smooth. Pour this mixture into the bottom of the prepared baking pan and spread it evenly.
Spread the bread cubes evenly over the brown sugar mixture in the pan, ensuring that they cover it completely.
In a medium-sized bowl, whisk together the eggs, milk, vanilla extract, and 1 teaspoon of cinnamon. Pour this egg mixture over the bread cubes, making sure every piece is saturated.
Sprinkle the remaining teaspoon of cinnamon and the tablespoon of brown sugar over the top of the bread mixture to create a sweet crust.
Place the baking pan in the preheated oven and bake for 25-30 minutes. The casserole should be golden brown on top and slightly puffed.
Remove the casserole from the oven. If desired, dust with powdered sugar before serving.
Enjoy your delicious French toast casserole!