Preheat your oven to 375°F (190°C). Generously grease a 9x13 inch baking dish with butter or non-stick cooking spray.

Open the cans of cinnamon rolls, setting both containers of icing aside for later use. Place the rolls onto a clean cutting board. Use a sharp knife to cut each of the 16 cinnamon rolls into four evenly sized pieces (quarters). Place all the cut pieces into a very large mixing bowl.

In a small bowl, melt the 4 tablespoons of butter. Once melted, stir in the ½ cup of light brown sugar and the 1 teaspoon of ground cinnamon until well combined. Pour this mixture over the cut cinnamon roll pieces in the large mixing bowl. Toss gently until the roll pieces are evenly coated with the brown sugar and butter mixture.

In a separate medium-sized bowl, whisk together the 4 large eggs, ½ cup of whole milk or half-and-half, 1 teaspoon of vanilla extract, and a small pinch of salt. Whisk until the custard mixture is fully blended and slightly frothy.

Pour the egg and milk custard mixture over the cinnamon roll pieces in the large mixing bowl. Fold the ingredients together gently once more to ensure all the roll pieces are coated in the liquid.

Transfer the entire mixture into the prepared 9x13 inch baking dish. Spread the pieces out evenly. Bake for 25 to 30 minutes, or until the edges are golden brown and bubbling, and the custard is fully set (a knife inserted near the center comes out clean).

Remove the casserole from the oven and let it cool for about 5 minutes. If the reserved icing seems too thick, whisk in a tiny amount of milk (about ½ teaspoon per container) to loosen it.

Drizzle the entire amount of icing evenly over the warm casserole. Serve immediately while warm.