Coat 8 (4-ounce) ramekins with cooking spray, and sprinkle with 2 tablespoons granulated sugar.
Combine 1/2 cup granulated sugar, cocoa, flour, and salt in a small saucepan. Gradually add milk, stirring with a whisk until blended. Bring to a boil over medium heat; cook until thick (about 3 minutes), stirring constantly. Remove from heat; let stand 3 minutes. Gradually stir in vanilla and egg yolks. Spoon chocolate mixture into a large bowl; cool.
Place egg whites in a large bowl; beat with a mixer at high speed until foamy. Gradually add 1/4 cup granulated sugar and cream of tartar, beating mixture until stiff peaks form. Gently stir one-fourth of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture and the chopped chocolate. Spoon into prepared ramekins.
Bake at 350° for 15 minutes or until puffy and set. Sprinkle with powdered sugar.
This is an absolutely incredible recipe: tastes horribly decadent, but is actually somewhat healthy. Followed the recipe to the tee, and got 10 full 4-ounce ramekins out of it -- maybe because my egg whites were incredibly stiff. Have not use cream of tartar in this application so may have made a bit of a difference as I usually have very stiff whites normally. For the best impression, definitely serve warm out of the oven so you get the absolute puffy souffle effect. Like any souffle, they deflate when cool. But they are yummy anyway!!!! The recipe above is missing either an added note to the directions or someone's review. I had in my notes that you could make up the mixture the day before and refrigerate overnight and bake for 22 minutes. I had only 7 ramekins and quite a bit of mixture left over, so saved the mixture and cooked the next day. The "leftovers" were absolutely just as good.