First, mix 300 g (2 ½ cups) flour, 100 g (½ cup) sugar, and a pinch of salt in a bowl. Then cut the butter into small cubes and knead it into the flour mixture with your hands until you have nice streusel/crumble. Thoroughly grease a 26 cm (10-inch) springform pan and press two-thirds of the streusel mixture into the pan to form a base and a roughly 5 cm (2-inch) high edge. Place this prepared springform pan in the fridge while you work on the filling.

Bring 1 liter (4 cups) of milk to a boil in a large pot. Once it begins to simmer, add the rum, cinnamon, vanilla sugar or vanilla paste, and lemon zest. In a separate bowl, mix the ground poppy seeds, semolina, cornstarch, and sugar. Carefully stir this mixture into the boiling milk and let it simmer on low heat for about two minutes. I like to add a tablespoon of soft butter at this point. Then, transfer the mixture to a bowl and let it cool for five minutes.

While the poppy seed mixture is cooling, drain the sour cherries well in a sieve. Preheat the oven to 180 degrees Celsius (160 degrees Celsius fan-forced, 350 degrees Fahrenheit). Stir the drained cherries and the egg into the poppy seed mixture. Pour this mixture into the prepared springform pan, smooth it out, and sprinkle the remaining streusel on top.

Now, bake the cake for about 50 minutes on the middle rack of the preheated oven. It will be hard to ignore the tempting aroma that will soon fill your kitchen! Once the cake is done, let it cool completely. If desired, you can dust the cake with some powdered sugar before serving.