Whisk the eggs, milk, vanilla, cinnamon, nutmeg, and salt together in a wide, deep bowl until the mixture is smooth and the spices are evenly dispersed with no streaks of egg remaining.

Set a griddle or large skillet over medium heat and allow it to warm up for a minute or two. Brush the surface with about 1 tablespoon of the butter, letting it melt and lightly coat the pan.

Working in batches, quickly dip the halved Hawaiian rolls into the custard mixture, cut side down first. Let each piece soak just long enough to absorb the liquid without becoming soggy, then gently shake off any excess before laying the rolls on the hot griddle.

Cook the rolls until the undersides are a deep golden brown and slightly crisp, about 2–3 minutes. Flip each piece carefully and cook the second side for another 2–3 minutes, until the centers feel set and the outside is evenly browned.

Transfer the cooked French toast to a plate or a baking sheet and keep warm (you can place them in a low oven if you like) while you repeat the process with the remaining rolls. Add more butter to the griddle as needed to prevent sticking and to keep the toast beautifully browned.

Once all the Hawaiian roll French toast is ready, arrange on serving plates, dust generously with confectioners’ sugar, and finish with a drizzle of warm maple syrup, if desired. Serve immediately while hot and fluffy.