Melt butter in a large saucepan over medium-high heat. Add carrots and onion, season with salt and pepper, and cook, stirring often, until carrots are softened, 15–20 minutes. Stir in broth, coconut milk, and 2 Tbsp. chili sauce. Bring to a boil, reduce heat, and simmer, stirring occasionally, until vegetables are very soft and liquid is slightly reduced, 40–45 minutes.