Preheat your oven to 375°F (190°C). Generously grease a 9x13-inch baking dish with the softened butter, making sure to coat the sides as well as the bottom.

Open both cans of cinnamon rolls. Using kitchen scissors or a sharp knife, cut each roll into 4 equal quarters. Arrange the pieces in the greased baking dish in a single snug layer — it's okay if they're touching.

In a large mixing bowl, whisk together the eggs, heavy cream, whole milk, brown sugar, vanilla extract, ground cinnamon, nutmeg, and salt until fully combined and smooth.

Pour the custard mixture evenly over the cinnamon roll pieces in the baking dish, making sure to coat all the pieces. Use a spatula or the back of a spoon to gently press any exposed pieces down into the custard.

Cover the dish loosely with aluminum foil. Bake covered for 20 minutes. Then remove the foil and continue baking for an additional 15–20 minutes, or until the top is deep golden brown and the center is just barely set (a slight jiggle is okay — it will finish setting as it rests).

Remove from the oven and let the casserole rest for 5 minutes.

While the casserole is still warm, warm the reserved icing packets in a small bowl of warm water for 1 minute to loosen them. Drizzle the icing generously over the entire casserole. Dust with powdered sugar if desired.

Scoop and serve immediately while warm, making sure each serving gets plenty of that gooey icing. Enjoy!