Heat 1 tablespoon olive oil in a large soup pot over medium-high heat. Cook prosciutto strips until crispy, 8 to 10 minutes. Drain on paper towels.
Heat remaining 1 tablespoon olive oil in the same pot over medium-high heat. Add celery, carrots, and onion; saute until vegetables begin to soften, about 10 minutes. Add butternut squash. Pour in chicken stock to cover vegetables (you will not use both boxes of stock here). Season with thyme, sage, cayenne pepper, salt, and pepper. Bring to a boil, reduce heat to low and simmer until vegetables are tender, about 30 minutes. Remove from heat.
Let soup cool slightly. Blend with an immersion blender. Add more chicken stock to thin to desired consistency.
Ladle soup into bowls and top with crispy prosciutto.
A 4-pound butternut squash yields about 7 to 8 cups of diced squash.
You may not use up both boxes of chicken stock. Use enough to cover the vegetables to cook them. Save the rest to thin out the soup if needed. It's easier to thin soup out at the end than thicken it up.
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