For the fruit filling: Pour in the fruit cocktail with the juice into a bowl and mix in the rum.
Preheat oven at 180°C/ 350°F. Cover the bottom of a 26-cm round springform baking pan with parchment paper.
Sift the flour, Cacao and baking powder in a bowl. Separate the eggs into 2 different mixing bowls.
For the sponge cake: Add a pinch of salt to the egg whites and beat at high speed with your mixer until it reaches the perfect foamy stiffness. Set aside. No need to wash beaters before using in the next step!
Add the vanilla extract, orange juice and sugar to the egg yolks and beat at high speed using the same beaters until the batter doubles in size.
Gradually add the flour to egg yolk mixture, a little at a time, folding just till blended. Next, carefully fold in third of the egg whites into the yolk flour mixture to lighten it up. Now fold in the rest of the egg whites and make sure not to knock all the air out. Pour batter into the prepared pan and bake for about 35 minutes. Do the toothpick test.
Let cool for 5 minutes, run a knife around the side of the pan to loosen the cake. Now carefully open the spring form. Place a wired rack on top of the cake and invert it. Remove the bottom of the pan and peel off the parchment paper. Let cool for at least 2 hours. At this point the cake can be tented with aluminium foil and stored overnight at room temperature.
Place the cake on the serving platter and slice it into 3 layers, either with a thread or a serrated knife.
For the filling: whip the cream and Sahnesteif until stiff peaks form. Drain the fruit cocktail over a colander reserving the juice. You are not going to need the whole juice! Take half of the whipped cream and mix it with the drained fruits. Reserve the other half for the frosting.
To assemble the cake: Lift the top 2 layers and place them on the rack. Drizzle about 4 tablespoons of the reserved juice all over the bottom layer. Take half of the fruit cream mixture and evenly spread it over the bottom layer. Cover with the second layer. Repeat as before with the juice and the rest of the filling. Place the top layer over the filling. With a fork poke a few holes all around the top. Drizzle 4 tablespoons of the juice over the top layer.
Last step: Frost the cake with the reserved whipped cream and garnish with the chocolate.