In your large slow cooker, add sweet potatoes, chilis, onion, garlic, pepper, salt, cumin, and broth. Cover and cook on high for 5-6 hours or low for 7-8 hours (potatoes should be tender).Using an immersion blender, or transfer contents of slow cooker into a stand blender, blend mixture until smooth. Add milk or cream, and blend a few more seconds to combine.Serve in bowls, garnished with a bit of sour cream if desired.