Wash the lemon hot, grate dry and thinly grate the peel. Halve lemon and squeeze juice. Cream 250 g butter, lemon zest and 275 g sugar with the whisk of the hand mixer. Add the eggs bit by bit. Mix flour and baking powder and alternately stir in lemon juice. Put the dough into the greased and floured fat pan of the oven (36 x 41 cm) and smooth it down. Bake the cake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 15 minutes.
Bring 1.3 litres of milk and 150 g of sugar to the boil. Stir the remaining milk and pudding powder until smooth. Remove the milk from the heat and stir in the pudding powder. Bring the milk to the boil again and simmer for about 1 minute, stirring continuously. Fill pudding in a bowl, cover surface directly with foil. Let the pudding cool down to room temperature
Whip 400 g room-warm butter until creamy with the whisk of the hand mixer. Stir the cooled pudding vigorously and add to the butter by the spoonful while stirring. Bring 5 tablespoons of water with 50 g sugar to the boil and let it cool down. Stir rum into the syrup. Chop the chocolate, bring the cream to the boil and stir in the chocolate. Pour chocolate cream into a freezer bag and cut off a small corner
Cut the cake in half crosswise and remove from the tray. Spread the cake halves one after the other with plum puree and butter cream. Dip the biscuits briefly in rum syrup and cover with cream. Decorate the cake with thin chocolate strips and chill for about 2 hours. Cut the cake into pieces. Whipped cream tastes good with it
waiting time approx. 2 1/2 hours