Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or cooking spray.
Prepare the King Cake: Cut your leftover King Cake into bite-sized pieces. If the cake is fresh and soft, let it sit out for a few hours to become slightly stale, which will help it absorb the custard mixture better.
Make the Custard: In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, sugar, vanilla extract, cinnamon, nutmeg, and salt until smooth and well combined.
Combine Cake and Custard: Place the cut-up King Cake pieces into the greased baking dish. Pour the custard mixture over the cake, pressing down gently with a spoon to ensure the cake pieces soak up the custard. If you’re using raisins or nuts, fold them into the mixture at this point.
Let it Soak: Allow the mixture to sit for 10-15 minutes so the King Cake can absorb the custard.
Bake: Cover the dish loosely with aluminum foil and bake in the preheated oven for 40 minutes. After 40 minutes, remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and the center is set (you can test by gently pressing the pudding to see if it jiggles).
Prepare the Icing: While the bread pudding is baking, mix the powdered sugar and milk in a small bowl to make a simple icing. Add more milk if needed to reach a drizzling consistency.
Serve: Once the bread pudding is done, remove it from the oven and let it cool slightly. Drizzle the icing over the top, and sprinkle with colored sugar in the traditional Mardi Gras colors of purple, green, and gold.
