Preheat oven to 160 C / Gas 3. Grease and line a loaf tin with parchment paper, allow a little bit of space either side to help lift it out.
In a bowl mix sugar, flour, bicarbonate of soda and cocoa powder.
Mash the bananas in a separate bowl, Stir in sunflower oil, one whole egg and milk. Separate the two other eggs and add the 2 egg yolks. Save the egg whites in a separate bowl.
Stir in the banana mixture into the dry ingredients. Whisk the egg whites until stiff peaks, then add a small amount of the whites to the main mixture to loosen the mixture. Then carefully fold in the remaining egg whites, careful not to knock any of the air out as you do, fold in until all the whites are mixed in.
Gently scrape into the tin and bake for 60 to 70 minutes. Leave to cool. Add some icing sugar on top to decorate!