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 degrees F, about 1-2 minutes. Remove from the heat; whisk in chocolate mixture. Set saucepan in ice and stir until cooled, about 5-10 minutes. Fold in whipped cream. Spoon into dessert dishes. Refrigerate for 4 hours or overnight.