Preheat oven to 350°.
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.