Step 1Peel the butternut squash and cut it into half-inch dice.
Step 2Heat the olive oil in a 6-quart stock pot. Saute the onion and leek over medium heat about 5 minutes, until just soft. Add the butternut squash and corn and saute for an additional 10 minutes. Add the rosemary and salt. Cover the vegetables with chicken or vegetable broth and cook uncovered for 30 minutes over low heat.
Step 3Strain and set aside half the stock; remove the rosemary branches. Puree the vegetables with an immersion or regular blender. Strain the puree through a large mesh strainer to remove the corn skin. Add half a cup of reserved stock (or more, if you prefer a thinner soup) back into the puree mixture. Store overnight in the refrigerator and reheat before filling an insulated container.