Put the flour, baking powder, powdered sugar, 5 egg yolks and cold butter in a bowl and knead into a mass. Then shape the dough into 3 balls of the same size. Mix a ball with the baking cocoa and knead again. Wrap a light-colored ball with the baking cocoa ball in cling film and place in the freezer for about 45 minutes.
Place the third ball in a baking pan as a base and press it down. Bake for about 10 minutes at 170 degrees top / bottom heat. Take out and let cool. Then coat the baked base with the jam. Remove the cocoa ball from the freezer and grate it into thick strips. Then pour this grated cocoa mass over the jam.
Now beat the egg whites firmly while slowly adding the sugar. At the end, fold in the cornstarch (or vanilla pudding powder instead). Distribute the protein mixture evenly on the cake. Take the last ball of dough out of the freezer, rub it too and finally cover the egg white mixture evenly.
Bake at 175 degrees top / bottom heat for 35 minutes. After the baking time, switch off the oven, but do not open it. Let the cake rest for 45 minutes. Then take it out and enjoy! As a decoration, the cake can be sprinkled with powdered sugar.