In the top of a double boiler set over simmering water, melt the chocolate, stirring, until it is smooth.
Remove the pan from the heat, add the egg yolks, one at a time, whisking well after each addition, and whisk in the rum and butter.
Transfer mixture to a large bowl.
In a bowl with an electric mixer, beat the egg whites with the cream of tartar and a pinch of salt until they hold stiff peaks.
Stir a quarter of the egg whites into the chocolate mixture then fold in the remaining egg whites gently but thoroughly.
Spoon the mixture into 6 buttered 2/3 cup souffle dishes and bake them on a baking sheet in the middle of a preheated 400 degree oven for 10-12 minutes, or until they are puffed and a tester comes out nearly clean.