Sift the flower; add the vanilla sugar and salt. Make a well in the middle and mix in the milk gradually until you have a smooth consistency without any lumps. Add the eggs and mix well.
Put the caster sugar in a plate. Peel the apple, cut in four and core. Cut the apple in thin slices; dip both sides in the caster sugar and set aside.
Melt a tablespoon of butter in a wide pan with a thick base and a low rim, on a medium low fire. Put in the apple slices in a nice pattern. Don’t be tempted to put too much apple because it would make it harder to flip the pancake over, half an apple will do, even less if you use a small pan. Let the apple get soft and the sugar caramelize, flip the apple over and pour the mixture on top. Make sure the apple pieces have the mixture on them but try to keep the pancake as thin as possible.
Wait until the bottom is browned and the top dried, no comes the only tricky part, the flipping over. Put some baking paper on a plate and slide the pancake from the pan on the plate. Hold the plate in one hand and the pan in the other, put the pan with the opening over the plate and turn over fast. Now you just have to wait until this side is brown and enjoy! Very nice with some cinnamon or vanilla ice cream. Time to make the next one, there is half an apple left.