The ingredients must be at room temperature.
Preheat the oven to 170 degrees.
Mix butter, honey, sugar, salt powder, vanilla sugar, baking powder, eggs, milk and flour in a bowl. We obtain an elastic dough that we will divide into 9 approximately equal parts.
Each part is spread on the floured work table, then place the sheet obtained on the tray greased with a little oil. We fit the sheet on the tray to cover the whole surface of the tray, we push it from place to place with a fork and we bake it for 10 minutes, until it starts to brown slightly at the edges.
We do the same with the other 8 sheets; after baking them all, place a large wooden chopper over the overlapping sheets, so that the sheets remain pressed until we fill them with cream.
Dissolve the starch and cocoa powder in cold milk (about 150 ml), mix well to avoid lumps.
The rest of the milk is put on the right heat in a pan. Add the sugar, vanilla sugar and the starch mixture with cocoa.
Stir continuously, so that no lumps form and the cream does not stick to the pan. After obtaining the consistency of a pudding, pull the pan aside and mix so that no crust forms over the cream, until it cools completely.
After it has cooled, beat the butter / margarine at room temperature, then add a tablespoon of butter to the boiled cream, beat at high speed until incorporated and then repeat the procedure until all the butter is incorporated into the cream.
With this cream we fill the sheets, then we place again a wooden shredder over the filled sheets, eventually we place over it and a weight of about 1-2 kg to press evenly the sheets filled with cream.
The next day, melt the dark chocolate in vegetable cream, grease the surface of the cake with chocolate (I also had some cream left over from the filling), sprinkle the candies with sugar, let the icing harden completely, portion and serve.