Melt semi sweet chocolate over a double boiler. Combine sugar and water in a saucepan. Boil to a temperature of 225 degrees. Beat egg yolks in an electric mixing bowl until creamy.
Gradually pour hot syrup over egg yolk mixture while whisking. Continue mixing until cool.
In a mixing bowl, whip heavy cream to a firm consistency. Fold cream and chocolate into the egg yolk mixture.
In a mixing bowl, whip egg whites with cream of tartar to a medium stiff consistency. Fold into chocolate mousse together with the cognac. spoon mousse into a clear glass serving bowl or into individual serving glasses. Refrigerate at least 2 hours before serving.
Garnish with whipped cream and chocolate curls.