Make a shortcrust pastry from the ingredients for the dough and wrap it in foil and let it rest in the refrigerator for 30 minutes.
In the meantime, grease and flour a springform pan (26 cm).
Finely crumble the rusks and mix with the ground almonds.
First roll out 3/4 of the shortcrust pastry into a round plate and place in the prepared pan.
Prick the base several times with a fork, spread the rusk-almond mixture on the base, place the blackberries on top and sprinkle with the sugar-cinnamon mixture.
Roll out the remaining shortcrust pastry into a round plate and carefully place on the berries, press down on the edges and prick the pastry lid several times with a fork.
Bake in the preheated oven at 190 ° C in the lower third of the oven for approx. 40 minutes.
For the topping, stir in the icing sugar and rum until smooth and pour onto the hot cake after baking.
Then let it cool down and serve with whipped cream.