Break the dark chocolate into small pieces and place in a heatproof bowl.

Separate the eggs, placing whites and yolks in different clean bowls.

Melt the chocolate with the water in a bowl set over a pan of simmering water, stirring until smooth.

Remove from heat and stir in the egg yolks one by one.

In a separate bowl, whisk the egg whites until stiff peaks form.

Gradually whisk in the caster sugar until the whites are glossy.

Gently fold a spoonful of the egg whites into the chocolate mixture to loosen it.

Carefully fold in the remaining egg whites until just combined.

Spoon into serving glasses and chill for at least 2 hours.