Prepare the Cherries and Quark: Drain the cherries if using canned. If using Greek yogurt, drain 1 kg in a colander lined with paper towels overnight in the fridge, then measure out 750 g. If using quark, let it drain in a colander for 10 minutes.
Make the Shortcrust Pastry: In a stand mixer, combine the flour, butter, sugar, vanilla sugar, and egg. Mix until the dough forms a ball. Wrap the dough in plastic wrap and chill in the refrigerator for 30 minutes.
Make the Custard Filling: In a large bowl, whisk together the quark (or drained yogurt), milk, egg yolks, melted butter (cooled), vanilla pudding powder (or cornstarch and vanilla seeds), and 130 g sugar until smooth. In a separate clean bowl, beat the egg whites with 60 g sugar until stiff peaks form, then fold them gently into the custard mixture.
Prepare the Crust: Remove the dough from the refrigerator. Press two-thirds of the dough into the bottom of a 9-inch springform pan, forming an even layer. Use the remaining dough to create a rim around the edges.
Assemble the Cake: Pour the custard filling over the crust and smooth the top with a spatula. Sprinkle the drained cherries evenly over the filling.
Bake: Preheat the oven to 350°F (175°C). Bake the cake for 60-70 minutes, or until the filling is set and the crust is golden brown.
Cool and Serve: Let the cake cool completely in the pan to ensure the filling sets. Once cooled, sprinkle with confectioners' sugar for presentation. Slice and serve. Enjoy!