Heat oven to 325°F Grease an 8-inch spring form pan and line the bottom with parchment paper; grease paper and set aside.
Melt chocolate, butter and water in the top of a double boiler set over simmering water, stirring to combine. Remove from heat and transfer to a large bowl; cool to room temperature. Add ¼ cup of the sugar substitute, the cocoa powder and vanilla extract to chocolate mixture, stirring until combined.
In a medium bowl, with an electric mixer on medium-high, beat eggs until mixture forms thick ribbons when beater is lifted, about 6 minutes. Reduce speed to medium; gradually add remaining ½ cup sugar substitute and beat until combined, 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 until evenly risen and almost set, 40-45 minutes (it will look like a brownie). Cool completely on a wire rack. To serve, run a knife around edge of pan and remove rim. Cut into 12 pieces and serve with whipped cream (optional).