Coat 2 (6 ounce) ramekins with cooking spray; sprinkle each with 3/4 tsp sugar.
Combine 2 T sugar, flour, cocoa, and milk in a small saucepan over medium heat. Cook 2 minutes, stirring until smooth. Spoon chocolate mixture into a medium bowl; cool 4 minutes. Stir in vanilla.
Place egg white in a medium bowl; beat with a mixer at high speed until soft peaks form. Add remaining 2 T sugar, 1 tsp at a time, beating until stiff peaks form (do not overbeat). Gently stir 1/4 of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture.
Spoon mixture into prepared dishes. Sharply tap dishes 2 or 3 times to level. Place dishes on a baking sheet; bake at 350 for 15 minutes or until puffy and set.
Sprinkle each souffle with 1/2 tsp powdered sugar. Serve immediately.