Preheat the oven to 356°F (180 °C). Divide the sludge/runny dough into 2 parts - put 1/3 of it in 1 bowl and the remaining 2/3 in another. Combine the citric acid with the baking soda in a small bowl. Divide them up the same way, add the larger part to the larger dough mixture, set aside the other part for later (you have to add it immediately before you put the small layer to bake).
Pour the larger dough part into a greased form with removable sides that you've covered with baking paper. Bake the layer in the preheated 356°F (180 °C) oven until your toothpick comes out dry when you poke it. Take it out and leave it to cool on a cooking grid. Now add the citric acid and baking soda to the smaller mixture, stir and bake it the same way.
Preparation of the cream: Beat the pudding in 2/5 cup (100 ml) cold milk. Heat the remaining 3 1/3 cups (800 ml) on the stove and add the sugar and pre-whipped egg yolks to it. Don't stop stirring until the milk comes to a boil. Next, add the butter, which you've cut into small lumps. Stir until it melts completely. Leave the cream to cool, covered with foil, to prevent a crust from forming on top and in the meantime prepare the gelatin as described on the package. Add the gelatin to the now cooled cream and stir.
At the bottom of the form you're going to arranging the cake, place a thick piece of cardboard, wrapped in foil. Wrap the walls with foil as well. Cut the thick layer into 2 equal parts. Place one of them at the very bottom. Pour half the cream on top of it and place the other part of the cut layer, which you've cut or broken apart into small pieces.
Add the bananas and fig jam and pour cream on top again. At the very top, place the thin cake layer, which you've cut in the form of a flower (use a cup in the middle to cut out a stamen and relatively equal-sized pieces to form the petals).
Cut out 1/8" thin strips between the petals, to leave a little empty space between them and have the cream show through. Another option is to cut the layer into 12 identical slices and remove 2 of them. On top, decorate as desired with chocolate, fruits, cream or whatever you like. Leave the cake for 12 hours in the fridge to harden and soak in the juices.