Make the dough: In a mixing bowl combine the butter, sugar, egg, cocoa powder, flour, and salt. Mix until a soft dough forms.
Prepare the pan: Preheat the oven to 350F (180C). Grease a 10 - inch (26 cm) springform pan.
Form the base: Press about two-thirds of the dough into the bottom of the pan and slightly up the sides to create a small edge. Set the rest of the dough aside for later.
Add the cherries: Drain the sour cherries well and spread them evenly over the crust.
Prepare the filling: Beat the butter and sugar until creamy. Add the eggs one at a time. Mix in the quark and vanilla pudding powder until smooth.
Assemble the cake: Pour the quark filling over the cherries. Crumble the remaining dough and sprinkle it over the top as streusel.
Bake: Bake for 50-55 minutes, or until the top is golden.
Cool: Let the cake cool completely before removing it from the pan. Dust with powdered sugar before serving if desired.