Stir in ½ cup of coconut milk -- or, if unconcerned with fat content, or a richer soup, add more, or most of the can, reserving some the harder "cream" for the garnish.
Puree soup until very smooth. Either blend in small batches in a blender or use a submersion blender. Get it as smooth as you can. Return to the stove and heat on med low.
Bring to a boil and add yams and optional carrot. Simmer uncovered until yams are fork tender, 15-20 minutes. Let cool a bit, while you prepare your garnishes.
In a large heavy bottomed pot heat coconut oil over medium high heat. Add shallot (or onion) and stir for 2 minutes. Add ginger, garlic, lemon grass, and chili, turn heat to medium and cook for 3-5 minutes, until fragrant and golden. Add spices, and stir for 1-2 more minutes. Add stock.