Bring a small saucepan of water to a simmer. In a bowl add the the chocolate and butter and sit it over the simmering water without letting it touch the water. Stir until melted, then set aside to cool slightly.
In another bowl add egg yolks, sugar, vanilla and water and also sit over the simmering water and whisk until pale; about 3 mintues (it will still be somewhat runny).Add to the chocolate and stir to combine.
Whick your egg whites until thick and glossy. (but not super stiff like you would with meringue)
Add a third of the egg whites to your chocolate and incorportate well. Add the remaining egg whites and fold though until well incorported. Add to invidiual pots or a large bowl and refridgerate.