Put the butter in a large, heavy-bottomed pot and melt it over medium-high heat.
Add the onion and celery, and season with salt and pepper.
Sauté the vegetables until they are softened but not browned, which takes approximately five minutes.
Add the curry powder, stir, and cook for two more minutes.
Add the broth to the pot.
Bring to a boil over high heat, then lower the heat and let simmer until the vegetables have completely softened, which takes about 10 to 12 minutes.
Add the carrots and ginger and simmer for an additional 30 minutes.
Using a blender carefully puree the soup in batches, and then return the soup to the pot over medium heat.
Season with salt and pepper.
Stir in the crème fraîche.
Reheat -- but be careful not to let the soup return to the boil.
Divide the soup among individual bowls and serve at once, topping each serving with a drizzle of crème fraîche, if desired.