Add the diced sweet potatoes and stir in the broth, cumin, chili flakes, and ginger.
Bring the mixture to a boil, then reduce the heat to low. Let it simmer until the sweet potatoes are tender, about 20 minutes.
Melt the butter in a large pot over medium heat. Add the onion and garlic, cooking until they're soft and fragrant.
Season with salt to taste.
Serve hot, garnished with a dollop of sour cream or a sprinkle of fresh cilantro if desired. Enjoy this spicy sweet potato soup with a slice of crusty bread or a side salad!
Use an immersion blender to puree the soup until smooth. For extra creaminess, stir in coconut milk.