Melt butter in a large pot over medium-high heat. Add carrots and onion, season with salt and pepper, and cook, stirring often, until carrots are softened, about 15-20 minutes.
Stir in broth, coconut milk, and chili sauce. Bring to a boil, reduce heat, and simmer, stirring occasionally, until vegetables are very soft and liquid is slightly reduced.
Let soup cool slightly, then purée until smooth. Add ginger and season to taste with salt and pepper. Serve and top with cilantro.