Prepare the cherries: Drain canned sour cherries in a colander for at least 10 minutes. If using fresh cherries, pit them using a cherry pitter or small knife.
Make the shortcrust pastry: In a stand mixer with paddle attachment, combine 250g flour, 125g cold butter, 150g sugar, vanilla sugar, and 1 egg. Mix until dough forms a cohesive ball. Shape into disk, wrap, and chill for 30 minutes.
Prepare the custard filling: In a large bowl, whisk together 750g quark, 100ml milk, 2 egg yolks, 25g melted butter, vanilla pudding powder, and 130g sugar until smooth and creamy.
Whip the egg whites: In a clean bowl, whip 2 egg whites with 60g sugar until stiff peaks form. Gently fold into the quark mixture to lighten the custard.
Assemble the cake: Press two-thirds of chilled dough into bottom of 9-inch springform pan. Use remaining third to form a rim around the edge. Pour custard filling into crust and scatter 500g drained cherries evenly over top.
Bake: Bake at 350°F (175°C) for 60-70 minutes until top is golden and center is set with slight jiggle. Cool completely in pan before slicing.
Serve: Dust with powdered sugar before serving. Store covered in refrigerator for up to 4 days.