Put the rusks in a large freezer bag and crumble them finely with a rolling pin. Melt the butter in a small pan and add it to the rusk crumbs. Add 125 g sugar and 5 tablespoons cold water and mix well. Grease a springform pan (26 cm Ø) and press the rusk crumbs onto the bottom and the edge (2-3 cm high). Put the dish in a cool place for about 20 minutes. Mix eggs, remaining sugar, vanilla sugar and salt with the whisk of the hand mixer until thick and creamy (approx. 5 minutes).
Mix quark, cream cheese, flour, orange and lime peel and orange juice and stir into the egg foam mixture. Pour the cream cheese mixture on the rusk base into the springform pan, smooth it down and bake in the preheated oven (electric cooker: 150 °C/ gas: level 1) 2nd rail from the bottom for approx. 30 minutes. Then turn up the oven temperature (electric oven: 175 °C/ gas: level 2) and bake for another 45 minutes (cover the cake with parchment paper if necessary). Remove from the oven, carefully remove the cake from the edge with a sharp knife and let it cool in the mould on a grid. Then remove from the mould and dust with icing sugar. Serve decorated with lemon balm, orange and lemon slices