For the pastry: pile flour on the work surface, mix with salt and and make a well in the center. Cut cold butter into small pieces and arrange around the well. Add egg and about 2 tablespoons of cold water and chop all ingredients wtih a knife until crumbly. Quickly knead into smooth pastry and shape into a ball. Wrap in plastic wrap and refrigerate for about 30 minutes.
For the crumble: combine all ingredients and process until crumbly. Refrigerate, covered.
Roll out pastry between 2 sheets of baking paper. Place pastry into a lined with parchment paper baking pan.
For the topping: rinse, halve and pit plums. Separate eggs. Whisk quark with sugar, vanilla sugar, egg yolks, cornstarch, butter, orange juice and orange zest. Beat egg whites until stiff and fold into cream mixture. Spread cream on cake and arrange plums on top. Sprinkle with crumble.
Bake in preheated oven at 180°C (approximatley 350°F) for about 60 minutes. If necessary, cover with buttered parchment paper.
Remove from the oven and cool. Dust with powdered sugar and serve.