Cut off and discard top of lemongrass, leaving a 6-inch stalk, then smash stalk with side of a large heavy knife.  Cook lemongrass, onion, and 1 teaspoon salt in butter and oil in a heavy medium pot over medium-low heat, stirring occasionally, until onion is softened, about 10 minutes. Add squash, corn, water, 1 ½ teaspoons salt, and ½ teaspoon pepper and bring to a boil over high heat.  Reduce heat and simmer, covered, stirring occasionally, until squash is tender, about 10 minutes. Remove from heat and discard lemongrass.  Purée soup in 3 or 4 batches in a blender until very smooth (use caution when blending hot liquids), straining each batch as blended through a fine-mesh sieve into a large heatproof bowl, pressing hard on and then discarding solids. Season with salt and pepper and reheat if necessary.