Preheat oven to 350°.
Coat a 2-quart soufflé dish with cooking spray; sprinkle with 2 teaspoons sugar. Set aside.
Combine 1/2 cup sugar, flour, and cocoa in a medium saucepan; gradually add milk, stirring with a whisk until blended. Cook over medium heat 5 minutes or until thick and bubbly, stirring constantly with a whisk.
Beat yolks in a large bowl with a whisk. Gradually add chocolate mixture to yolks, stirring constantly with a whisk. Stir in banana, margarine, rum, and vanilla. Set aside.
Beat egg whites and salt at high speed of a mixer until foamy. Gradually add 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold one-fourth of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture. Spoon into prepared soufflé dish.
Place soufflé dish in a 9-inch square baking pan; add hot water to pan to a depth of 1 inch. Bake at 350° for 55 minutes or until puffy and set. Remove from water; let cool to room temperature. Cover and chill 8 hours. Top each serving with 1 tablespoon whipped topping.