Peel and roughly chop the onion. Shuck the corn and cut the kernels off the cobs with a large knife. Place the butter in a pot and melt over medium heat. Add the onion and sauté until soft and translucent but not beginning to brown, about 8 minutes. Add some salt and the corn, stir and cook for another 3 minutes. Cover with 1 qt of water, turn up the heat and bring to a boil. Turn the heat back down and simmer until the corn is just soft, 3 to 5 minutes. Remove from the heat and purée in a blender until smooth, in batches if necessary. Depending on the power of your blender, you might want to pass the soup through a fine sieve (this isn't necessary with my Vitamix). Taste for salt and serve.