Melt butter over medium heat in a large, heavy bottomed pot; Stir in rice to coat grains.
Add 2 ounces (1/4 cup) milk, to cover the rice by roughly 1/4 inch. Stir rice every minute or so, until milk has been absorbed and mixture turns very thick. Be sure to occasionally scrape the bottom and sides of the pan so the rice doesn't stick. Repeat this step until all milk has been incorporated.
Stir in eggnog, spices, and raisins, and cook until rice is tender to your liking and the desired consistency is reached. (I cooked to al dente, which gives the rice a little bite.) For a thinner pudding, add a little more milk or eggnog.
Remove from heat and stir in vanilla.
Garnish with a sprinkle of nutmeg and a few more raisins. Serve warm or cold.