In bowl, mix powdered whites and water; let stand.
In large metal bowl, combine yolks, 1/4 cup sugar, coffee and butter. Place over saucepan of simmering water. Cook, stirring, until temperature reaches 160°F on instant-read thermometer, 5 minutes. Remove bowl from saucepan. Add chocolate; whisk until melted. Stir in brandy. If little pieces of cooked egg appear, strain through sieve into clean bowl.
Beat reconstituted powdered whites until soft peaks form. Gradually beat in remaining 1/4 cup sugar until stiff, moist peaks form. Fold quarter of whites into chocolate mixture. Fold in remaining whites. Refrigerate, covered, until mixture just begins to mound, 1 hour. (Or pour into 1-1/2-quart serving bowl and refrigerate at least 3 hours or until firm.) Spoon into six 8-ounce serving glasses; refrigerate 2 hours, until firm. (Can be made a day ahead; refrigerate.) Garnish with whipped cream, if you wish.