In a microwave or double boiler, heat chocolate, 1/4 cup water and butter until the chocolate and butter are melted. Cool for 10 minutes.
In a small heavy saucepan, whisk egg yolks, sugar and remaining water. Cook and stir over low heat until mixture reaches 160°F (70°C), about 1 to 2 minutes.
Remove from the heat; whisk in chocolate mixture. Set saucepan in ice and stir until cooled, about 5 to 10 minutes.
Fold in whipped cream. Spoon into dessert dishes. Refrigerate for 4 hours or overnight.