Mix flour, 200 g sugar, 150 g rock candy, vanillin sugar, salt and baking powder in a mixing bowl. Add eggs and buttermilk and mix with the whisk of the hand mixer to a smooth dough.
Pour the dough onto a well greased fat pan of the oven (32x39 cm, 3-3.5 cm deep). Mix the rest of the sugar and candy. Spread evenly on the dough. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 30 minutes.
Then remove from the oven, place on a grid and immediately spread the cream evenly over the still warm cake. Let the cake cool down and then dust it thinly with icing sugar as desired.
Whipped cream tastes good with it. Makes about 25 pieces.