Preheat oven to 175C/350F
Place the egg whites in a clean mixing bowl and whisk on medium/high until soft peaks form.
Slowly add the maple syrup; 1 spoonful at a time (adjust accordingly if making lemon or orange)
Mixture will triple in size and form peaks.
Add the vanilla extract (if making vanilla cake add extra vanilla now also) and whisk on medium/high until combined.
Add egg yolks, 1 at a time, whilst still whisking. Add the next yolk only once the previous one has been combined.
Add the milk whilst still whisking.
If making lemon/orange flavour add the juice/rind now.
In a separate bowl, sift the coconut flour and baking powder together.
Add the dry ingredients to the wet and whisk on low until combined (you may need to scrape down the edges to ensure the coconut flour is fully combined).
Turn the whisk to medium/high again and whisk for 30-60 seconds to make a fluffy mixture.
Pour into a greased and lined 20cm/8 inch cake tin.
Bake for 33-35 min, or until skewer comes out clean.
Allow the cake to cool before removing from the tin.