Melt the butter in a saucepot over medium heat and add the rice, toast a minute or 2, and then stir in the stock. Bring to a boil, reduce the heat and cook, covered, over low heat, about 15 minutes.
To a food processor, add the spinach, cilantro, if using, and a little salt, and pulse into a fine paste. Stir the paste into the rice after 15 minutes. Turn off the heat, cover the pot and let the rice stand another 5 to 10 minutes. Fluff with a fork.