Preheat your oven to 375°F (190°C). Lightly butter a 9x9 inch (23x23 cm) square casserole pan.
Tear ½ loaf of bread into bite-sized pieces (about 1-inch chunks). Distribute the bread pieces evenly in the prepared casserole pan.
In a large bowl, whisk together the 5 large eggs, 1 ½ cups milk, ¼ teaspoon ground cinnamon, and a pinch of salt until well combined and frothy.
Carefully pour the egg mixture evenly over the bread pieces in the casserole pan. Gently press down on the bread to help it absorb the liquid. Let it sit for 5-10 minutes to soak.
While the bread soaks, melt 4 tablespoons of butter. Stir in the 3 tablespoons of packed light brown sugar and ¼ teaspoon ground cinnamon until thoroughly combined.
Drizzle the melted butter and brown sugar mixture evenly over the soaked bread in the casserole pan.
Bake for 40-45 minutes, or until golden brown, puffed, and the center is set.
Remove from the oven and let rest for 5 minutes before serving.