Melt the butter in a large pot or Dutch oven over medium heat. Add the diced butternut squash, carrots, and onion. Cook, stirring occasionally, for about 10 minutes until the vegetables begin to soften.
Add the brown sugar, salt, curry powder, nutmeg, cinnamon, and optional cayenne pepper to the pot. Stir continuously for 1 minute until the spices are fragrant.
Pour in the pumpkin purée, apple cider, and vegetable broth. Stir everything together until well combined.
Increase the heat to bring the soup to a boil. Once boiling, reduce the heat to low, cover, and let it simmer for 20-25 minutes. The soup is ready for the next step when the squash is very tender and mashes easily with a fork.
Remove the pot from the heat and stir in the coconut milk.
Use an immersion blender to purée the soup directly in the pot until it is completely smooth. Alternatively, let the soup cool slightly and blend in batches in a traditional blender. Only fill the blender halfway and cover the lid with a towel to be safe.
Taste the soup and add more salt if needed. Serve warm.