Use 1 tablespoon of butter to grease your cocottes. Sprinkle them with cocoa powder, tapping off the excess. Place cocottes on a baking dish and set aside.
In a double boiler, melt chocolate with the remaining butter. Whisk smooth.
Set aside to come to room temperature. Add egg yolks, one at a time and whisk well.
In a clean bowl of your stand mixer, with a clean whisk, beat egg whites with a dash of salt, cream of tartar and sugar on medium speed until frothy. Increase the speed to high and beat until stiff peaks form. Do not over beat. Keep checking the peaks. As soon as you see the stiff peaks form, stop.
Add ⅓ of the meringue into chocolate mixture. Gently fold in. Add the remaining meringue and fold in until no streaks remain. Do not over fold.
Spoon the mixture into prepared cocottes. At this point you can cover the dishes and refrigerate the souffles until ready to bake.
Place in the oven and bake for 20 minutes.
Sprinkle with powdered sugar and serve immediately.
Notes
It is crucial to beat the egg whites in a clean bowl with a clean whisk. There can be no traces of grease or the meringue won't form. Bake the souffles right before you are ready to serve the dessert for the maximum impression.