Preheat oven to 375 degrees F.
In a large metal bowl set over a pan of hot, not simmering, water (bottom of the bowl should not touch the water), combine the chocolate and butter and let stand, stirring occasionally, until smooth and melted. Learn different techniques for How To Melt Chocolate.
In a medium-size bowl, combine the 6 egg yolks, sugar, and Splenda. Add the melted chocolate/butter mixture and vanilla extract and stir to combine. Be sure to add the beaten eggs to the chocolate mixture and not the chocolate to the eggs. This will insure a moist airy texture.
In a large bowl, whip the 6 egg whites until firm. Add 1/3 of the whites to the chocolate mixture and stir to lighten the batter. Gently fold in the remaining egg whites until just blended and no streaks remain. Finish by using a rubber spatula to ensure that the heavier mixture at the bottom is incorporated.
Scrape the batter into a non-stick 10 springform pan and smooth top with the spatula.
Bake approximately 20 minutes or until just set (do not overcook). Remove from oven and let cool. Cake will collapse some while cooling.
To Unmold: Run a thin metal spatula around the side of the cake and release the sides of the springform pan. The chocolate cake will store up to 2 weeks refrigerated. Do not freeze because it changes the texture.
When ready to serve, bring to room temperature. Cut into narrow wedges with a thin sharp knife that has been dipped in hot water. Dust with sifted powdered sugar before serving.
Makes 12 to 16 servings.
* I sometimes substitute a coffee liqueur for the vanilla extract. Be creative!
Perfect Flourless Chocolate Torte Recipe: https://whatscookingamerica.net/desserts/flourlesschoccake.htm