Position oven rack to the lowest setting, and remove middle rack. Preheat oven to 425°.
To prepare the soufflés, lightly coat 6 (8-ounce) soufflé dishes with cooking spray. Sprinkle evenly with 2 tablespoons sugar. Set aside.
Combine remaining 1/2 cup sugar, 3 tablespoons flour, 3 tablespoons cocoa, and salt in a medium saucepan over medium-high heat, stirring with a whisk. Gradually add 1 1/4 cups milk, stirring constantly with a whisk; bring to a boil. Cook 2 minutes or until slightly thick, stirring constantly with a whisk; remove from heat. Add 3 ounces chocolate; stir until smooth. Transfer mixture to a large bowl; cool to room temperature. Stir in vanilla and egg yolk.
Place egg whites in a large mixing bowl; beat at high speed with a mixer until stiff peaks form (do not overbeat). Gently fold one-fourth of egg whites into chocolate mixture; gently fold in remaining egg white mixture. Gently spoon mixture into prepared dishes. Sharply tap dishes 2 or 3 times on counter to level. Place dishes on a baking sheet; place baking sheet on the bottom rack of 425° oven. Immediately reduce oven temperature to 350° (do not remove soufflés from oven). Bake 40 minutes or until a wooden pick inserted in the side of soufflé comes out clean.
To prepare sauce, melt butter in a small saucepan over medium-high heat. Add 1/3 cup sugar, 2 tablespoons cocoa, and 1 tablespoon flour; stir well with a whisk. Gradually add 1/2 cup milk, stirring well with a whisk; bring to a boil. Cook 1 minute or until slightly thick, stirring constantly with a whisk. Remove from heat; add 1/2 ounce chocolate, stirring until smooth. Serve warm with soufflés.