Thinly slice the potatoes and onion.
Heat butter in a pot and sauté the vegetables.
Add in the curry powder and continue to sauté. Pour in water and boil for 7 minutes, until the potatoes are cooked.
Turn off the heat and pour in milk. Stir well and add in the heavy cream.
Using a blender, blend the mixture to smoothen.
Transfer the soup back into the pot and simmer in low heat. DO NOT let it boil, or the ingredients may separate.