1) For the crępes: In a medium-sized bowl, whisk together the flour, salt and cocoa. If the cocoa has any lumps be sure to break them up or sift them out.
2) Whisk in the eggs, milk and butter and continue to whisk until the mixture is smooth. Pour the batter through a strainer into a clean bowl, cover and set aside for 30 minutes.
3) Towards the end of the rest period, preheat your crępe pan according to the manufacturer's direction. Cook a test crępe or two, and adjust the batter with milk if it is too thick to work with. Cook the rest of the crępes, you'll need about 18 to 20 for the cake.
4) Cool the crępes while you prepare the pastry cream filling. A pastry cream filling mix is a quick shortcut, or you can make your own from scratch. You will need approximately 2 cups of cooled pastry cream.
5) To make hazelnut pastry cream: Warm the chocolate hazelnut spread in the microwave for 20 seconds, or until it is soft and almost pourable. Whisk the warm spread into the pastry cream, and continue to whisk until it is lump free.
6) To assemble the cake: Stack 3 crępes on a serving plate and spread with 2 tablespoons of hazelnut pastry cream. Continue to layer a single crępe and 2 tablespoons of filling until you are nearly out of crępes. For stability, occasionally layer two or three crępes instead of one single crępe.
7) End with a layer of 3 crępes. Wrap the cake well in plastic wrap, using a little pressure to keep the cake pressed down. Chill for at least 60 minutes, or up to overnight. Slice in wedges to serve.