Boil 800 ml milk. Stir 2 sachets of custard powder, 100 g sugar and 100 ml milk until smooth. Stir into the boiling milk, bring to the boil and simmer for about 1 minute. Remove from the stove. Place in a bowl and cover directly on the surface with foil. Leave to cool at room temperature for about 6 hours. Allow 375 g butter and quark to reach room temperature in about 6 hours.
Melt the rest of the butter. Meanwhile separate the eggs. Beat the egg whites and salt with the whisks of the hand mixer until stiff, adding 150 g sugar at the end. Beat the egg yolks one after the other.
Mix flour and baking powder, sieve onto the egg mixture and fold in the melted butter. Spread the mixture evenly on a baking tray (approx. 34 x 40 cm) lined with baking paper. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 10-12 minutes. Take out of the oven, remove the sponge cake from the edge, turn onto a tea towel sprinkled with 2 tablespoons of sugar, remove the baking paper. Let the sponge cake cool down.
Wash the strawberries, dab dry, cut some in half and put aside for decoration. Clean the rest of the strawberries and dice them. Stir 2 sachets of custard powder, 100 g sugar and 100 ml juice until smooth.
Bring 300 ml juice and berry mixture to the boil in a saucepan. Stir in the mixed juice, bring to the boil and simmer for about 1 minute, stirring several times. Add the strawberries and let cool down. Halve the sponge cake.
Shortly before it becomes firm, distribute and spread the groats evenly on the floors. Let it cool down.
Remove foil from the pudding. Stir the pudding briefly with a whisk. Whisk the warm butter with the whisk of the hand mixer until thick and creamy. First stir in the pudding, then the quark slowly by the spoonful.
Spread the butter cream evenly on the groats and smooth it down. Dip the biscuits briefly in rum. Place them close together on the cream. Place in a cool place for about 30 minutes. Cut into pieces and dust with icing sugar. Decorate with strawberries.