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. Also allow 375 g butter and quark to reach room temperature in about 6 hours.
Meanwhile, for the dough, whisk 250 g butter, salt, vanillin sugar and 200 g sugar with the whisk of the hand mixer until thick and creamy. Add the eggs one by one and stir in. Mix flour, starch and baking powder, alternately add 2-3 tbsp. milk and stir in.
Pour the dough onto a greased fat pan (approx. 34 x 37 cm) and smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 18-20 minutes. Remove from the oven, place on a cake rack and let it cool down.
Peel or clean, wash and chop the vegetables. Peel and finely chop onion and garlic. Wash thyme, shake dry, remove leaves. Bring 3-4 l salt water (1 teaspoon salt per litre) to the boil.
Spread evenly on the cooled down floor. Allow to cool.
Remove foil from the pudding. Stir the pudding briefly with a whisk. Whip 375 g 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. Place in a cold place for about 30 minutes. Then place the biscuits in a checkerboard pattern on the cream (approx. 5 x 7 biscuits).
Cut into pieces and if necessary dust thinly with cocoa.