Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2). Grease a box form (30 cm long; approx. 2 1⁄2 l capacity) and sprinkle with breadcrumbs. To measure sugar, flour, etc., use a cup that holds 180 ml of liquid.
Beat the eggs, sugar, vanilla sugar and 1 pinch of salt with the whisk of the hand mixer for about 8 minutes until foamy.
Mix hazelnuts, cocoa-containing drink powder, cocoa, flour and baking powder. Stir into the egg mixture. Stir in the oil and fizz bit by bit.
Fill the dough into the mould. Place in the preheated oven, turn down the oven temperature (electric oven: 150 °C/circulating air: 125 °C/gas: level 1) and bake the cake 1 1⁄4-1 1⁄2 for hours. Make a cooking test towards the end: Prick wooden sticks at the highest part of the cake until the middle.
If no moist dough sticks, the cake is cooked. Remove the cake and let it cool down for about 20 minutes. Then turn out of the tin and let it cool down on a cake rack.
Roughly chop or break the chocolate into pieces and melt it with plate-fat in a hot water bath while stirring. Leave to cool slightly. Roast hazelnut flakes in a pan without fat. Take out and let cool down.
Spread the chocolate icing on the cake. Sprinkle hazelnut flakes over the cake after about 10 minutes. Let the chocolate icing get firm.