Step 1 Preheat your oven to 325°F (165°C) and grease a 9x13-inch baking dish with 2 tablespoons of unsalted butter to prevent sticking.
Step 2 In a large mixing bowl, whisk together 3 cups of whole milk, 2 large eggs, ½ cup of granulated sugar, 1 teaspoon of vanilla extract, ½ teaspoon of ground cinnamon, until the mixture is smooth and well combined.
Step 3 Stir 2 cups of cooked rice into the custard mixture. If desired, fold in ½ cup of raisins or other mix-ins like dried cranberries or chopped nuts for added flavor.
Step 4 Pour the rice and custard mixture into the prepared baking dish, spreading it evenly. Place the baking dish into a larger roasting pan and fill the roasting pan with hot water until it reaches halfway up the sides of the baking dish.
Step 5 Bake the pudding in the preheated oven for 1½ hours, or until the top is golden and a knife inserted in the center comes out clean.
Step 6 Carefully remove the baking dish from the water bath and let the pudding cool for 15-20 minutes before serving warm, or refrigerate for at least 2 hours to serve chilled.