Preheat oven to 190 degrees Celsius Coat 1/2 litre souffle dish with cooking spray and sprinkle lightly with caster sugar.
Combine cocoa with 90 ml hot water in a bowl. Set aside.
Melt margarine in a saucepan, add flour and cook 1 minute, strring constantly. Gradually add milk and 50 g sugar, stirring until thickened. Add salt and remove from heat. Add cocoa mixture, and stir well to combine. Let cool slightly for a few minutes.
Whisk egg whites until stiff, but not dry. Add remaining sugar, a little bit at a time, mixing after each addition. Gently fold in cocoa mixture, and spoon into prepared souffle dish.
Back in preheated oven 35 minutes or until risen and set. Remove fromoven and serve warm. Serve with sweetened Greek yogurt (or thick cream if you want to ignore the healthy thing), if desired.