In large bowl, mix powdered egg whites and water; let stand.
In large metal bowl, combine yolks, 1/4 cup of the sugar, the cream and butter. Place over saucepan of simmering water. Cook, stirring constantly, until temperature reaches 160 degrees F on instant-read thermometer, about 4 to 7 minutes. Remove bowl from saucepan. Add chocolate; whisk until melted, replacing over saucepan if needed. If little pieces of cooked egg appear, strain through sieve into clean bowl.
Beat reconstituted powdered whites until frothy. Gradually beat in remaining 1/4 cup sugar until stiff, moist peaks form. Fold one-quarter of the beaten whites into chocolate mixture. Fold in remaining whites. Refrigerate, covered, until mixture just begins to mound when spooned, 1-1/2 hours. Spoon into six 8-ounce serving glasses; cover and refrigerate another 1-1/2 hours, until firm. Or pour into a 1-1/2-quart serving bowl and refrigerate 3 hours or until firm. (Can be made a day ahead; refrigerate until ready to serve.) Garnish with whipped cream and crushed cookies, if desired.