Peel butternut squash. Cut in half lengthwise and scoop out seeds. Cut flesh into cubes.
Melt 2 tablespoons butter and coconut oil in a soup pot over medium-high heat. Add onion and cook until softened, about 3 minutes. Add squash, red peppers, red curry paste, ginger and garlic. Mix thoroughly and cook for 5 to 7 minutes. Add extra butter if the pot dries out, to avoid burning onion or squash.
Pour coconut milk and skimmed milk into the pot; bring to the boil. Reduce heat and simmer soup, stirring often, until squash is soft, about 25 minutes.
Pour 1/3 of the soup into a blender. Cover and hold lid down; pulse a few times before leaving on to blend. Pour into a large bowl. Repeat twice more.