Heat the oil in a medium saucepan over medium heat. Add the leeks and cook, stirring, until tender, 3 to 5 minutes. Add the potato, bay leaf, garlic, rosemary, 1/2 teaspoon salt and a few grinds of pepper and cook, stirring, until the potatoes are well coated, 2 to 3 minutes. Pour in the broth and 1 cup of water and bring to a simmer.
Add the butternut squash and simmer until the vegetables are tender, about 20 minutes, adjusting the heat as needed to maintain a simmer. Remove from the heat, and let cool at least 5 minutes. Remove and discard the bay leaf and rosemary and carefully puree until smooth with a handheld immersion blender or in 2 to 3 batches in a blender (do not fill blender more than halfway full).
Add the cream and chopped rosemary and reheat the soup if necessary. Adjust the consistency and seasoning with water, salt and pepper. Serve hot and garnish with more rosemary leaves and a drizzle of olive oil.
Note: When blending hot liquid, first let it cool for 5 minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters and pulse until smooth.
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