Bring water and sugar to a boil and let boil for 3 minutes.
Place egg yolks in mixing bowl and slowly pour in sugar/water mixture.
Mix until pale yellow and thickened.
In a stainless steel bowl, set over a pot of hot (not boiling) water, melt chocolate and butter.
Whisk melted chocolate into egg mixture until blended.
In a separate bowl, pour the cream then whip until stiff.
Add the whipped cream to the chocolate mixture and gently fold together until no streaks remain.
With a spoon or a piping bag, fill cups or ramekins 3/4 full. Refrigerate for at least 2 hours.