Preheat oven to 375°.
Coat a 1 1/2-quart soufflé dish with cooking spray; sprinkle with 1/4 teaspoon sugar. Set aside.
Combine cocoa and hot water in a bowl. Stir well; set aside.
Melt margarine in a small, heavy saucepan over medium heat. Add flour; cook 1 minute, stirring constantly with a whisk. Add milk, 1/4 cup sugar, and salt; cook 3 minutes or until thick, stirring constantly. Remove from heat. Add cocoa mixture; stir well. Spoon into a large bowl; let cool slightly.
Beat egg whites (at room temperature) at high speed of a mixer until foamy. Add 3 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold 1 cup egg white mixture into cocoa mixture; gently fold in remaining egg white mixture. Spoon into prepared soufflé dish.
Bake at 375° for 35 minutes or until puffy and set. Remove from oven; serve warm, at room temperature, or chilled with Warm Turtle Sauce.