In a saucepan combine the water and sugar, bring to a simmer, stirring, over moderate heat until sugar is dissolved. Remove for the heat, add the chocolate and stir until smooth. In another saucepan sift the cocoa powder. Whisk in the half-and-half in a stream. Place over moderate heat and bring to a simmer, whisking. Add the melted chocolate to the pan, whisking, until hot. Do not boil. Off the heat whisk in the vanilla and cognac, if desired. Pour into cups and garnish with the whipped cream and chocolate shavings. Serve immediately.