Heat a large pot or Dutch oven under medium high heat. Add cooking oil, onions and garlic. Saute for a minute or two until onions soften. Avoid browning onions and garlic.
Add butternut squash, carrots, and chicken broth. Bring to a boil. Partially cover with lid, and cook on medium/medium low heat for 15 minutes, or until butternut quash and carrots are softened.
Add curry powder, brown sugar and salt. Stir to combine. Turn off heat.
Using an immersion blender, blend butternut squash and carrots in broth until soup is smooth and creamy. If blender is stuck, add more chicken broth as needed. If you don't have an immersion blender, soup can be transferred to a regular blender or food processor to blend as well.
Heat soup on medium heat, add coconut milk. If you want to add coconut milk swirls to your curry butternut squash soup at the end, save a few tablespoons of coconut milk for later. Stir, bring soup to a boil, and cook for another 3-5 minutes, or until it reaches your desired consistency.
Taste and add more salt to open up the flavors if necessary.
Drizzle with coconut milk (optional). Serve with croutons, and and enjoy!