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.
You can prepare the ingredients ahead, spoon the batter into soufflé dishes, cover, and freeze until you're ready to cook them. They can go straight from the freezer to the oven. Make the sauce ahead, too, and simply warm it before serving. This recipe received our Test Kitchens' highest rating.