For the sponge cake: 1st variant (the faster one, takes about 45 minutes): Make a sponge dough with 4 eggs, 180 g sugar, 200 g flour and some baking powder.
The easiest way is to beat the 4 eggs with the sugar until very foamy and then stir in the flour with some baking powder. Then fill half of the dough into each of two springform pans lined with baking paper (with rim!)
and bake at about 180 degrees convection.
Depending on your oven, the baking time can vary, 10-15 minutes is normal for me. Finger test: press the dough slightly with one finger, if the dough rises again on its own, it is ready. Repeat this process with another 4 eggs, the remaining sugar, flour and some baking powder.
Once the dough has cooled down, cut the resulting 4 pastry cases once again to obtain 8 pastry cases. Important: take your time and work accurately!
If you make a mistake, it is not so bad, the dough is anyway ‘stuck together’ with the cream, broken parts are not noticeable. 2nd variant (for those who have a lot of time, takes 1-2 hours, depending on practice): Make the sponge dough from 1 egg, 45 g sugar, 50 g flour and some baking powder and pour it into a springform pan (with rim) lined with baking paper. Spread evenly. I do best with a kitchen spatula, as you can use it to work exactly to the edge. Do the same again, and fill the 2nd springform pan. Then put both springform pans in the oven and bake at 180 g convection oven for 5-10 minutes (depending on the oven).
In this variant, check more often when the dough is ready. Then repeat the same procedure until all 8 bases are baked.
You could also make the dough of 2 eggs at once and then divide it into 2 parts.
However, I personally prefer making one at a time rather than dividing (for accuracy).
For the cream, take 5-6 tablespoons of milk to mix the custard powder and stir the egg into the rest. Then bring the milk (with egg) to a boil and stir in the stirred custard powder (without sugar!) and finish cooking the custard according to package directions. Stir the coarsely chopped dark chocolate into the still hot pudding and let it melt completely.
Cover the pudding with plastic wrap to prevent a skin from forming and let cool.
When the pudding and butter are at room temperature (it takes a few hours!) cream the butter and stir in the pudding by the tablespoonful. Assembling the cake:
Put the first cake layer on a cake plate, soak it with some rum, spread some cream on it (roughly 2-3 tbsp), then put the 2nd layer on top and proceed as with the first until all layers are on top of each other.
Spread cream on the last base as well, and level out the edge with the remaining cream (there’s not much left).
Melt the dark chocolate coating in a water bath and cover the cake with it from top to bottom.
If possible, use a knife to score the pieces of cake in the slightly dried chocolate coating, this makes cutting the cake much easier! It tastes best when it can infuse at least 1 night.