Knead butter, 75 g sugar and 1 egg with the dough hooks of the hand mixer. Add 20 ml milk in a thin stream. Then knead in flour. Wrap dough in foil and chill for about 1 hour.
Roll out the dough on a floured work surface to a thickness of approx. 4 mm. Line a tart mould (Ø 28 cm) with lift-off base with the dough and press the edges lightly.
Wash the selected blackberries and dab dry. Clean, wash, grate dry and stone the cherries.
Separate 1 egg. Mix egg yolk, quark, 70 g sugar, oil, 250 ml milk and pudding powder with a whisk to a smooth mixture. Then stir in lemon grated lemon and salt, add the quark pudding mixture into the tart. Use the egg white for other purposes.
Spread the blackberries and cherries on the tart and press them in slightly. Bake the tart in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 1 hour. Then let it cool completely on a cake rack. Lift the tart out of the tin, place it on a cake plate and dust the outer edge with icing sugar.