In a heavy-bottomed saucepot, bring the milk, cinnamon stick, and scraped seeds from the vanilla bean (or the vanilla bean paste) up to a simmer. Add the rice and cook at a gentle simmer, uncovered and stirring often, until the rice is al dente, about 20 minutes. Remove from the heat and stir in the raisins (if using) and butter to melt. Remove the cinnamon stick.