Preheat oven to 400°F (200°C). Cut the butternut squash in half lengthwise, scoop out the seeds, and drizzle with olive oil. Place cut-side down on a baking sheet and roast for 40-50 minutes, or until tender.
While the squash is roasting, sauté the onion in a pot over medium heat until softened, about 5 minutes. Add garlic and ginger; cook for 1 minute more.
Once the squash is cooked, scoop out the flesh and add it to the pot with the onions. Add chai spice, vegetable broth, salt, and pepper. Bring to a boil, then reduce heat and simmer for 15 minutes.
Using an immersion blender, or in batches in a regular blender, blend the soup until smooth. Stir in coconut milk.
Serve hot, garnished with fresh cilantro.
Notes
Feel free to adjust the amount of chai spice to your liking. For a creamier soup, add more coconut milk.