Whisk the egg yolks and sugar in a bowl until they whiten. Break the chocolate into pieces. Melt it with a tablespoon water, in a bowl fitted into a saucepan with 1-inch simmering water in it. Stir with a wooden spoon until the chocolate is smooth. Off the heat, stir in the butter. Add the melted chocolate to the sugared egg yolks. Beat the egg whites with the salt until firm. Fold the egg whites into the chocolate mixture with a spatula. Pour into a bowl and refrigerate for at least 3 hours.
