Beat the fat, sugar, salt and vanilla sugar until creamy with the whisk of the hand mixer. Add the eggs bit by bit. Mix flour and baking powder and stir it into the fat egg mass by the spoonful.
Add lemon zest. Halve the biscuits and fold into the dough. Grease a ring cake tin (2 litres capacity) and sprinkle with breadcrumbs. Fill in the dough and smooth it down. Bake in the preheated oven (electric: 175 °C/ gas: level 2) for about 50 minutes.
Let it rest on a cake rack for about 10 minutes. Then turn over, sprinkle with orange juice and let it cool down completely. Stir icing sugar and lemon juice until smooth and spread over the cake. Sprinkle with chopped candied lemon peel and leave to dry.
Results in about 16 pieces.