Combine chocolate chips, sugar and water in top of double boiler, set over hot (but not boiling) water. Cook till chocolate is melted.
Beat yolks in small bowl until thick and lemon colored (1 1/2 min). Add to chocolate mixture and stir 3 min. Transfer to large bowl.
Beat egg whites till stiff but not dry. Fold into Chocolate, fold in whipped cream.
Slice cake into thin wedges. Arrange enough slices in bottom of 10" springform pan to cover completely. Spread with some of the chocolate filling. Repeat layering, ending with filling. Refrigerate overnight.
Just before serving, remove sides of springform pan. Top cake with remaining whipped cream. Sprinkle with chocolate.