Preheat the oven to 350 degrees.
In a medium saucepan, over a medium heat, melt the butter. Cook the onion until translucent. Add garlic and cook for 30 seconds. Add the corn kernels and salt, then cover the pan and reduce the heat. Cook until the kernels are tender, about 2 to 3 minutes. Add the epazote. Set aside to cool.
Slit the chiles down the sides and carefully remove the seeds, leaving the top and stalk intact.
Stuff the chiles with the corn mixture. Put a slice of the cheese in the center of the filling.
In a shallow ovenproof dish, place the chiles over the Arroz Blanco, gently pushing them into the rice. Pour the crema over them and bake until heated through, about 15 to 20 minutes. Sprinkle with the grated Manchego cheese and continue to bake just until the cheese is melted.
Whisk the cream and the buttermilk together in a bowl. Cover and set in a warm place (a gas oven with just the heat from the pilot light is fine) for 8 hours. Crema may be kept in the refrigerator for as long as a week.
Rinse the rice in cold water until water runs clear. Let the rice drain for a few minutes.
In a medium size saucepan with a tight-fitting lid, heat the oil over a medium low heat. Add the onion and cook until translucent, stirring constantly. Add the garlic and cook for 2 minutes. Add the rice and cook just until the grains are coated with the oil. Add the water and cook, uncovered, over a medium heat until the liquid has been absorbed and small air holes appear on the top of the rice. Do not stir. Wrap the lid of the saucepan with a small terry cloth dish towel. Place the lid tightly over the saucepan so that none of the steam can escape. Set aside in a warm place for about 20 to 30 minutes, so that the rice can continue to cook and the grains expand.
Stir the rice well before serving.
c.1997, M.S. Milliken & S. Feniger, all rights reserved