To begin, line the bottom of a 26 cm (10 inch) springform pan (or a cake tin with a removable base) with parchment paper. Then, grease the sides of the pan with butter.
In a bowl, combine an egg, sugar, butter, flour, and baking powder. Mix all the ingredients together using your hands, a kitchen machine, or a hand mixer with dough hooks until a firm dough is formed.
Next, divide the dough into two portions: one-third (1/3) and two-thirds (2/3). Take the two-thirds (2/3) portion of the dough and crumble it evenly into the prepared springform pan (or cake tin).
With floured hands, press the dough down to cover the bottom of the pan, making sure to also create a slightly raised edge along the sides. Set this aside. The remaining portion of the dough will be used later.
Mix the vanilla seeds with sugar and mix very well. Add half a tablespoon of vanilla sugar you just made to a bowl with rhubarb and mix well.
Next preheat the oven to 180°C (356 °F) with both up and low heat (Convection bake function: 160 °C (320 °F)).
In a small pot or saucepan, combine the milk, the remaining vanilla sugar, and cornstarch (cornflour). Stir continuously over medium heat until the mixture starts to bubble and thickens.* If you feel that the pudding is not sweet enough to your liking, feel free to increase the amount of sugar. Once thickened, remove the pot from the heat and let the pudding cool for 2 to 3 minutes. After the pudding has cooled for a few minutes, add the Schmand (or whichever alternative you are using) and mix thoroughly until well combined.
Spread the prepared pudding mixture over the cake base, ensuring it covers the entire surface. Use a spoon or spatula to smooth the pudding layer.
Next, scatter the rhubarb pieces evenly over the pudding mixture. Then, crumble the remaining dough evenly over the top of the rhubarb layer.
Bake the cake in the preheated oven for approximately 50 minutes, or until the top is golden brown. Once baked, remove the cake from the oven and allow it to cool completely before serving.
Hint: I prefer to bake this cake before going to bed so that it can steep overnight, as it tends to taste even better this way. However, it is also delicious when baked and consumed on the same day.