Combine the chocolate and butter and melt over a hot-water bath.
Combine the egg yolks with the water and 1 ounce sugar and whisk over a hot-water bathe to 145 degrees F for 15 seconds. Remove from the heat and whip until cool.
Combine the egg whites with the remaining 1 ounce sugar and whisk over a hot-water bathe to 145 degrees F. Remove the whites from the heat and beat to full volume. Continue beating until cool.
Using a large rubber spatula, fold the egg whites into the egg yolks.
Fold the butter-chocolate mixture into the egg mixture.
Fold in the whipped cream. Add the rum, to taste.