Preparation Whisk egg yolks, 1/4 cup sugar, unsalted butter and 1/4 cup water in large metal bowl to blend. Set bowl over saucepan of simmering water; whisk constantly until thermometer inserted into mixture registers 160°F, about 3 minutes. Add chocolate and whisk until melted and smooth. Turn off heat; leave bowl over water. Beat egg whites in another large bowl until soft peaks form. Gradually add remaining 1/4 cup sugar and beat until stiff but not dry. Fold 1/3 of beaten whites into chocolate mixture to lighten. Fold in remaining whites. Divide mousse among 6 small dessert glasses. Chill until set, at least 3 hours or overnight.
Preparation Whisk egg yolks, 1/4 cup sugar, unsalted butter and 1/4 cup water in large metal bowl to blend. Set bowl over saucepan of simmering water; whisk constantly until thermometer inserted into mixture registers 160°F, about 3 minutes. Add chocolate and whisk until melted and smooth. Turn off heat; leave bowl over water. Beat egg whites in another large bowl until soft peaks form. Gradually add remaining 1/4 cup sugar and beat until stiff but not dry. Fold 1/3 of beaten whites into chocolate mixture to lighten. Fold in remaining whites. Divide mousse among 6 small dessert glasses. Chill until set, at least 3 hours or overnight.