For the topping, bring the fat, sugar, oat flakes and 4 tablespoons of water to the boil while stirring and allow to cool slightly. For the sponge mixture, stir the fat and vanillin sugar until foamy. Add eggs bit by bit.
Stir in syrup, lemon peel and juice. Mix flour, baking powder and hazelnuts and fold into the mixture. Grease a springform pan (24 cm Ø) and fill in the dough. Smooth the oatmeal topping on it and bake in a preheated oven (electric range: 175 °C/ gas: level 2) for approx. 75 minutes.
Remove and let it cool down. Results in about 12 pieces.