make the dough
Add flour, sugar, baking powder, butter (cut into cubes), and eggs to a bowl. Knead the butter into the flour mixture until no lumps or chunks of butter are left. Place in the fridge for 20 - 30 minutes.
make the poppyseed filling
Add milk, sugar and butter (cut into cubes) to a saucepan. Place over medium heat and bring to a boil. Add the ground poppy seeds and stir into the mixture. Add the semolina and cook over low heat for 3 - 4 minutes while stirring regularly. Set aside and stir in the whipping cream, rum, lemon zest, and sour cream. Stir to combine, set aside and bring to room temperature.
Stir the Graham cracker crumbs into the room-temperature poppyseed filling. When combined, add the eggs and stir to combine.
Grease a 20 cm x 25 cm (8-inch x 10-inch) cake pan with butter. Sprinkle your working surface with flour. Add two-thirds of the dough on top. Store the rest of the dough in the fridge. Roll the dough into a 20 cm x 25 cm (8-inch x 10-inch) rectangle. Transfer to the prepared cake pan with a rolling pin. Spread the poppyseed filling evenly on top. Grate the rest of the chilled dough on top of the filling. Sprinkle with Graham cracker crumbs.
German Poppy Seed Cake
Place the German Poppy Seed Cake into a preheated oven. Bake for one hour at 180 °C / 350 °F. When the cake is baked, transfer it to a wire rack to cool, then cut it into slices and serve.