Place the cooked rice, 1 ½ cups of the milk, granulated sugar, and salt into a nonstick saucepan. Heat the mixture over medium heat, bringing it to a gentle boil, and let it simmer uncovered for 15 to 20 minutes, stirring often until the texture becomes rich and creamy.
In a separate mixing bowl, beat the egg together with the remaining ½ cup of milk. Slowly whisk approximately 1 cup of the hot rice mixture into the egg mixture, ensuring it’s combined smoothly. Then return this tempered mixture to the saucepan. Stir in the golden raisins and continue to cook for an additional 2 minutes.
Take the pan off the heat and fold in the butter, vanilla extract, cinnamon, and nutmeg until fully incorporated.
Serve warm, garnished lightly with additional nutmeg and cinnamon if desired.