Prepare the ingredients. Open both cans of cinnamon rolls and remove the icing packets. Reserve the icing for use during cooking and for the final topping.

Prepare the slow cooker. Coat the interior of a 6-quart slow cooker with nonstick cooking spray or lightly grease it with butter.

Cut the cinnamon rolls. Slice each cinnamon roll into four equal pieces, then distribute the pieces evenly across the bottom of the prepared slow cooker.

Whisk the eggs. Crack the four eggs into a mixing bowl and whisk thoroughly until evenly blended.

Prepare the custard mixture. Add the milk, ground cinnamon, vanilla, and maple syrup to the beaten eggs. Whisk until the ingredients are completely incorporated.

Combine the casserole. Pour the egg mixture evenly over the cinnamon roll pieces, taking care to coat the pieces throughout.

Add the first icing packet. Drizzle the contents of one reserved icing packet evenly across the top of the casserole mixture.

Slow-cook the casserole. Cover the slow cooker and cook on the LOW setting for 2 to 2½ hours. Check the casserole periodically toward the end of cooking to ensure the edges do not become overly browned while the center cooks completely without remaining soggy.

Finish with icing. Once the casserole is fully cooked, remove the lid and drizzle the second reserved icing packet over the surface before serving.