Melt the butter and set aside.
In a large mixing bowl mix the flour, salt, orange zest and sugar. Make a well in the centre, add the egg and gradually pour in half of the milk whilst gently stirring. When you get a smooth batter, add the remaining milk and the melted butter and which until you get a silky, smooth batter resembling single cream. If you have time, rest the batter for up to an hour.
Heat a heavy-based frying pan or crêpe pan. Add a dice of butter and grease the pan all over. Use a piece of paper kitchen towel to remove carefully the excess oil and put it aside.
Pour a small ladleful of batter into the pan (about 2½ Tbsp) and tilt the pan to let the batter move around and spread evenly and thinly. Cook on medium heat for about a minute until the edges of the crêpe start to turn golden. Then turn or flip it over and cook for another 30-60 seconds until golden. Slide it onto a plate.
Use the oily kitchen paper to grease the pan and repeat the crêpe-making process until you used up all the batter.
Now make the sauce: mix the lemon & orange juice, the grated lemon zest and orange liqueur in a small jug. Pour half of this into a frying pan, then add the sugar. Cook it for two minutes at a low temperature until you get a light, brown sauce. Add the diced butter.
When melted, pour in the remaining citrus mixture and let it bubble, for 2-3 minutes, until you get a glossy sauce.
Fold each crêpe into quarters and lay up to four of them into the pan, creating a circle. Heat them through for a minute, while basting them to make sure the sauce coats them well. You can serve them immediately or you can flambé them in the pan.
To flambé, pour over the brandy and carefully ignite with a long match or taper. When the flames have died down, serve the crêpes with the caramelized sauce, sprinkled with some orange zest and dusted with icing sugar.