Coat a 9-inch springform pan with butter-flavored cooking spray.
3
In bowl of an electric mixer, beat egg whites on high until stiff, glossy peaks form.
4
Microwave chocolate bar in microwave-safe bowl on high for 1 to 2 minutes. Stir after each 30 seconds until smooth.
5
Scrape into a mixing bowl. Stir in cocoa, nuts, Splenda, sour cream, egg substitute, and vanilla extract. With a spatula, fold in egg whites. Do not completely stir in egg whites; batter should look marbled with white.
6
Spoon batter into prepared pan and gently smooth top. Bake for 30 minutes and cool.
7
Loosen edges of pan and remove cake. Cake will deflate.
8
Serve warm with low carb ice cream or fresh whipped cream, or simply dust with icing sugar.