Preheat oven to 325 degrees Fahrenheit.
Combine chocolate and butter in upper bowl of a double boiler. Melt slowly over simmering water. When melted thoroughly combine. Remove from heat.
In a standing mixer, beat egg whites at high speed until foamy. Add 1/2 cup granulated sugar. Continue to beat, until soft peaks form and hold. Remove from mixing bowl and place aside.
Place egg yolks into cleaned mixing bowl and beat at high speed, gradually adding remaining sugar. Continue to beat until the mixture triples in volume, the color is light yellow, and all the sugar has dissolved. Add the chocolate-butter mixture and beat until thoroughly blended.
Add the flour and beat at low speed, 10 seconds. While the mixer machine is running add egg whites. When thoroughly combined, detach the bowl and scrape along the sides and bottom to ensure all the mixture is fully incorporated.
Pour the mixture into a 9" x 1" cake pan. Fill to the rim and level the top. Place the cake pan into a deeper dish or roasting pan. Pour boiling water into deeper pan until it reaches halfway up the cake pan. Cover the deep pan with aluminum foil, leaving 2 inches above the cake pan to allow for rising.
Place into preheated oven and bake, 45 minutes. The cake will swell and expand 1"-2" above the cake pan rim, and deflate upon cooling.