1) Preheat oven to 350°F. (175°C.)
2) Put chocolate, butter, and sugar in saucepan. Set in skillet of simmering water. Stir until chocolate is melted or in Pyrex bowl and microwave on medium.
3) Using a food processor, whip egg yolks until light and lemon colored.
4) Add the hot chocolate sauce to form chocolate mousse. This will look like very thick mayonnaise.
5) Beat egg whites to soft peaks stiff but not “brittle.” If too stiff they will not be easy to fold into the mousse mixture.
6) Add egg whites to chocolate, starting with a quarter of the mixture and fold until incorporated. Add a second quarter; fold then add the rest producing a light mouse.
7) Use two 8 inch round cake pans that have been greased and set with a circle of folded waxed paper on bottom. Use the bottom of the pan to trace diameter on the folded waxed paper, and cut two at once. Spray Pam is perfect to secure the paper on pan bottom.
8) Reserve 1/2 of the light chocolate mouse. This will be used as filling and frosting. Place the remaining 1/2 in the two round cake pans.
9) Bake for 20 minute, toothpick comes out clean and should double in height. Upon removal, the cake will collapse.
10) Let stand 10 minutes, invert and cake should fall out with no coaxing. Remove the top wax paper and let cool.
11) Place half of the reserved mixture on top of first cake. Place 2nd cake on top to form layer, and use the remaining mixture to finish.
12) You may want to sift about a tablespoon of confectioner's sugar on top for a finishing touch.