Separate eggs; place yolks and whites in large mixing bowls and let stand at room temperature for 30 minutes. Meanwhile, preheat oven to 375 degrees F. Use 2 tablespoons of the butter to grease ten 4 ounce ramekins. Coat the ramekins with the cocoa, tapping out the excess.
In a medium bowl over a pan of simmering water (or a double boiler), melt the chocolate and remaining 4 tablespoons butter. Stir occasionally until smooth. Remove from heat and add the vanilla.
Lightly whisk together the egg yolks. Gradually whisk some of the chocolate mixture into the egg yolks. Add remaining chocolate mixture and whisk until smooth. Set aside.
Beat the egg whites and cream of tartar with an electric mixer on medium to high speed until soft peaks form (tips curl). Gradually add the granulated sugar, 1 tablespoon at a time, beating until stiff peaks form (tips stand straight). Gently fold about one - fourth of the beaten egg whites into the chocolate mixture. Add chocolate mixture to remaining beaten whites and fold the mixtures together. Spoon into prepared ramekins.
Bake for 20 to 25 minutes or until a knife inserted near the center comes out clean. Serve immediately. Sprinkle each serving with powdered sugar.
Note: For the most intense chocolate flavor, use a good - quality chocolate such as Ghiradelli.