Put the carrots into a saucepan with the water, bring to a boil, then decrease the heat to maintain a low simmer. Cover and cook for about 15 minutes, or until the carrots are very tender. Drain the carrots into a sieve placed over a bowl and reserve the liquid. Put the carrots into a blender with the butter, salt and sugar and process, adding enough of the reserved liquid to make a smooth puree. Season to taste with more salt if necessary and freshly ground pepper. Set aside or refrigerate and reheat before serving.