In a large bowl, mix the flour, sugar, vanilla sugar and a pinch of salt, then form a well in the center.
In a separate bowl, beat the eggs with the milk using a whisk. Gradually pour the mixture into the center of the well, whisking constantly.
Continue mixing, working in circles from the center outwards to avoid lumps. You should obtain a very smooth batter. If lumps persist, use an immersion blender to remove them.
Gradually add the hot melted butter, stirring well after each addition to ensure thorough incorporation.
Divide the batter into two equal parts. Sift the cocoa powder and incorporate it into one of the portions. Mix thoroughly to obtain a homogeneous batter.
Leave both batters to rest for an hour. The batter will thicken slightly.
After the resting time, mix the batter again, then add the water to make it more fluid. This makes for lighter crepes.