In a large soup pan over medium-high heat add the butter and onion. Stir until the onions are well-coated, and allow to saute until translucent, a few minutes. Stir in the curry paste, and then the carrots. Allow to cook another minute or two, and then add the coconut milk, salt, and water, adding more water to cover if needed. Allow to simmer until the carrots are tender, 10 – 15 minutes, and then puree using a blender or hand blender until the soup is completely silky smooth.
Add more water if the consistency needs to be thinned out a bit. After that taste for salt, adding more if needed. If you like season this soup with a great big squeeze or lemon or lime juice. Serve topped with whatever you have on hand: crunchy almonds, sprouts and/or cilantro, chile oil.