Remove the zest from the orange using a hand grater and separate the egg whites from the yolks.
Mix together the flour, cocoa powder, sweetener (reserve 1 teaspoon for the drizzle) the orange zest, juice of 1/2 the orange and 5 egg yolks. If this mixture is a bit dry then you can add a little water to make it easier to mix in the whisked egg whites.
In another bowl whisk the egg whites until they form soft peaks.
Gently fold the egg whites into the flour and egg yolk mixture, until they are mixed evenly.
Pour the mixture into a 2lb loaf tin (we lined the bottom with baking parchment and gave it a quick spray with low calorie cooking spray just in case).
While the cake is cooking, mix the remaining sweetener with the rest of the orange juice and heat on low in a sauce pan.
When the cake is done (to check insert a knife and it should come out clean) remove it from the oven, turn the heat down to 100ºC then pour the orange syrup over the top of the cake and put it back in for another 10 minutes.
Leave to cool, and then cut into 8 equal slices.