Combine the chocolate and water in a saucepan. Place in a skillet of barely simmering water and heat, stirring as necessary, until chocolate melts.

Meanwhile, put the egg yokes and sugar in a bowl of an electric mixer (or another heatproof bowl). Add the creme de cacao and set bowl in another skillet of barely simmering water. Beat vigorously with a wire whisk until the sauce becomes thick, like a stiff custard. Remove from the hot water and let cool briefly.

Let the melted chocolate cool slightly (if it is too hot when added to the sauce, it will tend to overcook the eggs). Add the chocolate to the sauce and fold it in with a rubber spatula. Beat in the rum.

Whip the cream until stiff. Fold in the chocolate mixture.

Spoon the chocolate filing into the bake tart shell. Cool briefly, then chill in the refrigerator or freezer, depending on the time available.

Garnish with additional whipped cream, if desired.