Preheat the oven to 175C/350F.
Grease and line a 20cm/8 inch round cake tin.
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.
The mixture will triple in size and form peaks.
Add the vanilla and whisk on medium/high until combined.
Turn whisk to medium and add egg yolks, 1 at a time. Add the next yolk only once the previous one has been combined.
In a separate bowl, sift the coconut flour and baking powder together.
Gently fold the sifted dry mix into the egg mixture.
Add the juice/rind and milk and gently fold.
Turn the mixer to medium and whisk again for 1 minute.
Pour into prepared tin.
Bake for 20 min, or until skewer comes out clean.
Cool on a rack. If the cake is too big to fit easily into the trifle dish cut in half and trim until it fits. Place on top of the mousse layer.