1. In a medium saucepan, warm the chocolate, water, and butter over low heat, stirring, until melted and smooth.
2. Remove from heat and stir in the sugar then the egg yolks.
3. Fill a basin or the sink with a few inches of very cold water and beat the chocolate for a few minutes until the consistency of brownie batter.
(The original recipe said “blanchiment” which I thought meant “whiten” but when she did it, it didn’t whiten, or even really lighten at all. But I’d still do it.)
4. In a separate bowl, beat the egg whites until stiff. Fold the whites into the chocolate mixture until smooth.
5. Moisten well a muslin cloth or a clean (lint free) tea towel, and line a 9-inch (23 cm) round glass or earthenware baking dish such as a pie plate, with it, letting the sides overhang the edges over the dish.
(Note: Although she normally uses and recommends a round dish, we used a square baking dish which worked fine.)
6. Scrape the batter into the dish and smooth the top. Lift the sides of the damp cloth and fold it over the batter, enveloping it.
7. Let chill for 2 days before serving.