Sheets: Beat the egg whites well with a pinch of salt. Add the sugar one at a time and continue mixing until you get a strong foam, thick as meringue.
Walnuts, flour and baking powder mix well together, then pour over the foam in several batches. Mix with a spatula until the whole amount of walnuts is incorporated. It is divided into two equal parts.
Line the oven tray with baking paper and pour part of the composition. Level it and put it in the preheated oven at 180 degrees C, for 15-18 minutes or until the top is baked (the toothpick test is done). The same is done with the other composition. Allow to cool.
Meanwhile, prepare the cream. In a pot, mix the flour with the sugar and vanilla sugar. Pour the milk a little and mix with a whisk to mix. Transfer the pot to a low heat and let it boil until it thickens like a pudding, stirring constantly so that it does not stick to the bottom. Add the dark chocolate, broken into pieces, mix well until it melts and incorporate all the chocolate. Remove from the heat and allow to cool slightly then add the butter. Mix well until it is incorporated into the cream. Allow to cool.
Take the meringue sheets and cut them in half. Place half a sheet on a plate, grease with part of the cream, put the second sheet, grease with cream, the third sheet, grease with cream and the last sheet. Grease a thin layer of cream on top and sprinkle with chopped hazelnuts on top.
Portion and serve. Good appetite!