Whipped cream dusted with unsweetened cocoa powder (optional)
Heat oven to 325 F. Grease an 8-inch springform pan, and line bottom with parchment paper; grease paper, and set aside.
Melt chocolate, butter and water together in the top of a double boiler over medium-low heat, stirring to combine. Remove from heat, and transfer to a large bowl; cool to room temperature.
Add 1/4 cup sugar substitute, the cocoa powder and vanilla extract to chocolate mixture, stirring until combined. In the medium bowl of an electric mixer, beat eggs on medium-high until mixture forms thick ribbons when beater is lifted, about 6 minutes.
Reduce speed to medium; gradually add remaining 1/2 cup sugar substitute, and beat until combined, about 1 minute.
Stir one-third of the egg mixture into the chocolate mixture to lighten. In two additions, fold in remaining egg mixture until well combined. Pour batter into prepared pan, smoothing top.
Bake 40 to 45 minutes, until evenly risen and almost set, like a brownie.
Cool completely on a wire rack. Run a knife around edge of pan, and remove rim.
Serve with whipped cream dusted with cocoa powder, if desired.