Step 1Combine the porcini and water in a saucepan and bring to a boil. Reduce the heat and cook at a very gentle simmer for 20 minutes; the liquid will reduce by about one-fourth. Remove from heat and set aside to cool for 10 minutes.
Step 2Purée the mushrooms and liquid in a blender, then return the purée to the saucepan and stir in the onion powder, salt, butter, sugar and soy sauce. Simmer gently for 5 minutes.
Step 3Ladle the soup into bowls and garnish with crème fraîche.