Break and separate eggs. Put yolks aside into a glass or cup for later.
Put egg whites into a mixing ball and whisk until hard peaks form.
Pour yolks into the mixing bowl slowly one by one whilst whisking vigorously.
Peel banana and place it in a bowl then squash it with a fork.
Measure and grind flax seed. Mix dry ingredients, flax seed flour, chestnut flour and baking powder in a medium size bowl.
Fold dry ingredients and squashed banana alternately into the egg mixture.
Pour batter into a loaf baking form and put it into the 350ºF / 180ºC preheated oven.
Bake it for 45 minutes. Check it with a toothpick if it's baked properly. (Stick toothpick into the bread if it comes out clean it is ready. If not, leave it for another 5-10 minutes. Wait at least 10 minutes before slicing it up.