Preheat oven to 400 degrees F. Butter 2 1-cup souffle dishes and sprinkle with sugar.
In a metal bowl over a pot of hot, but not simmering, water melt the chocolate, stirring until smooth. Remove bowl from heat and whisk in the egg yolks, one at a time. Whisk in the rum and butter.
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 one fourth of the whites into the chocolate mixture and then fold in remaining whites gently but thoroughly. Spoon the mixture into 2 1-cup souffle dishes that have been buttered and sprinkled with sugar. Bake the souffles on a baking sheet in the middle of the preheated oven for 10 to 12 minutes, or until they are puffed.
Recipe courtesy of Gourmet Magazine