Preheat the oven to 325 degrees F. Grease an 8 " springform pan. Melt the chocolate over a double boiler and then let cool.

In an electric mixer, cream the butter and sugar together at low speed until mixed. Set butter/sugar mixture aside in a separate bowl. Remove beaters from your electric mixer, and clean thoroughly so that all residue from butter and sugar is completely cleaned off and then dry.

Replace beaters into your electric mixer and then continue. Beat the egg whites on medium until soft peaks form; set aside this mixture into a separate bowl. Clean mixer bowl and beaters again.

Mix the cooled chocolate and egg yolks in the mixer on low speed until smooth and then add the reserved butter-sugar mixture and continue mixing until smooth. Increase the speed to medium and mix for another 2 minutes.

Gently fold in your egg white mixture, doing 1/3 at a time, until all incorporated.

Pour the batter into your prepared springform pan and bake for 20-30 minutes, or until a toothpick comes out almost clean and just barely moist. Remove your cake from the oven and let cool for 20-25 minutes before removing from springform pan. Slice cake into 10 wedges and serve.