Knead the butter, sugar, flour, whole egg, egg yolk and lemon zest into a dough.
Roll out to the width of a palm, lay it in a tray.
Brush the edge of the dough with egg white, and line it with a thick rolled strip of dough as a border, so it sticks.
Part-bake at 180°C, about 10-12 minutes, then let cool.
Filling: mix the ground hazelnuts, the 120 g sugar, the 50 g grated chocolate, the water and the 20 g flour, and spread over the dough.
For the top layer: heat the crushed sugar, 50 g hazelnuts and 50 g grated chocolate over a water bath, stirring constantly, then spread hot over the dough.
Return to the oven and bake until done, about 15 more minutes.
Once cool, cut into bars.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.