Preheat the oven to 200C/400F/Gas 6.
Place the flour, egg yolks, milk and orange juice and zest into a clean bowl and whisk until the ingredients have combined and the mixture has a smooth texture.
Place the egg whites into a separate clean bowl whisk them until stiff peaks are formed when the whisk is removed.
Add the caster sugar to the egg whites and continue to whisk the mixture until it has a glossy texture.
Fold the egg white mixture into the egg yolk mixture and stir until the two mixtures have combined.
Melt the butter in a large frying pan.
Add a large ladleful of the mixture to the pan and fry for two minutes on each side, until the pancake is golden. Remove from the pan. Repeat this process with the remaining batter until all the mixture has been used.
To serve, place the pancakes in a pile on a clean plate and dust with icing sugar.