Melt the butter in a small pan. Add the milk, salt and sugar, stir well and turn off the heat.
Put both flours in a mixing bowl, make a well in the centre and pour in the vegetable oil and add the eggs. Mix the eggs and oil with a whisk, gradually bringing in flour from the sides until it begins to thicken. Add the milk mixture little by little until all has been incorporated and the batter is smooth. Finally, whisk in the water.
Pour the batter through a medium strainer into a bowl and refrigerate for at least two hours. (The resting time allows the batter to relax and the flour to absorb the liquids fully. Pancake batter may be made up to a day ahead and refrigerated.)
To cook, heat a 15-18cm/6-7in frying pan. Add a very little oil. When the pan is hot, pour in just enough batter to cover the base of the pan.
Allow to cook on one side for 1-2 minutes, flip over onto the other side and continue to cook, until speckled and slightly golden.
Slide onto a plate and keep warm while you make the rest of the pancakes. You can stack one on top of the other as they can be peeled apart later but are best eaten fresh off the pan.