In a big bowl, mix the flour, salt, and 1 3/4 cups milk, in that order. Whisk vigorously by hand until it is the consistency of heavy cream. Break eggs and fold them in with a whisk. Add the remaining 1 3/4 cups of the milk to the melted butter and pour into the mixture. Continue mixing the batter until it becomes shiny and smooth. Finally sieve the batter to get rid of any lumps. Leave to rest for 2 to 3 hours.
Dip a piece of kitchen paper in vegetable oil and grease a small frying pan. Ladle a spoonful of batter into the pan. With a flick of the wrist swivel the pan in order to get a nice even covering all over. A good crepe should be paper- thin. Cook on a high heat and flip it over when you see the edge turning golden brown. Remove crepes from pan and keep aside in a warm place.
Fold the crepes in 4 and put them into a deep frying pan. Spoon in enough orange butter to barely cover them and, on a high heat, reduce the liquid, turning the crepes over to make sure that they absorb the orange butter.
To serve, place the crepes on a dessert plate. Spoon the orange butter over them and garnish with a sprinkling of the zest.
Melt the butter in a big saucepan on a high heat. Add the sugar and cook for about 10 minutes until it caramelizes. It should be a golden color.
Combine the lemon with the orange juice and pour it in when the caramel is bubbling. Be careful not to burn the butter or yourself. Add Grand Marnier and continue whisking as the mixture curdles a little and let it cook for 10 minutes to reduce. Finally mix with an electric hand-mixer to ensure that the butter is fully integrated.
Peel the zest from the oranges and cut it very thinly. Blanch in boiling water 3 times and drain it to remove the bitterness. Put the zests in a saucepan with the syrup. Simmer over low heat for 20 minutes.