The day before, chop 500 g of couverture for the base and melt over a warm water bath, stirring occasionally. Remove, let it cool down a little. Put the liquid chocolate coating and oil in a bowl and mix well with the whisk of the hand mixer.
Add 40 ml milk, cocoa, sugar, vanillin sugar and salt and stir for about 2 minutes until creamy. Stir in the eggs one by one. Stir in flour briefly. Put the dough on a baking tray covered with baking paper (approx. 39 x 32 cm) and smooth it down.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 25-30 minutes. Remove from the oven and allow to cool.
The next day, chop 75 g white and 75 g dark chocolate separately and heat and melt each in 80 ml milk in a small pot. Put 250 g butter and 450 g icing sugar into 2 mixing bowls and whisk with the whisk of the hand mixer for about 5 minutes until light and creamy.
Stir 80 ml of milk into the light chocolate coating and 80 ml of milk into the dark chocolate coating one after the other, each time stirring quickly under 1 portion of butter cream. Chill for about 30 minutes so that they are spreadable.
Cut 4 equal rectangles (approx. 14x18.5 cm) from the chocolate base. Place a rectangular cake frame around 1 base. Spread 1/2 light cream on top. Place 2 cake base on top, spread 3 tbsp. dark cream on top.
Place the base on top and spread the rest of the light cream on top. Cover with 4th base. Chill the cake and the rest of the cream for about 2 hours.
Remove frame carefully. Stir the cream again and spread the cake all around. Carefully spread crunchy beads close together on the cake and dust with gold powder.