Watch the recipe video for detailed instruction.
Prepare the shortcrust pastry: Stir baking powder into flour. Knead the flour, sugar, vanilla sugar, cold butter (in pieces), and egg together by hand or in a food processor to form a smooth dough.
Divide the dough into two parts: ¾ is needed for the crust and ¼ for the crumble topping.
Shape 3/4 of the dough into a ball, then flatten into a disc. Wrap the dough in cling film and place it in the fridge - for about 30 to 1 hour. Put the crumble part into a bowl and store in the fridge.
After chilling, press ¾ of the dough into a lined springform pan (9 inch / 23 cm) lined with baking paper, pulling up an edge of approx. to a rim of 4 cm ( 1 1/2 inches).
Put the springform pan in the fridge and prepare the cheesecake filling. If the dough gets too soft when you handle it, cool it in the pan until it hardens a bit.
Prheat oven to 350F. Drain the liquid from the skyr (or quark, if using). Mix cream cheese and skyr with the sugar, then add the eggs one by one making sure to reduce the speed and only mix as long as needed to incorporate the eggs and also stir in the custard powder. Mix just until everything is combined.
My springform pan is 23cm (9 inch) in diameter and the baking time was just over an hour! With frozen raspberries you have to bake the cake at least 10 minutes longer. With a bigger or smaller springform pan it can be done earlier or later. With a 10 inch it might alreqady be done in 50 minutes.
First, bake the cake undisturbed in the 350 F/ 180 C hot oven for about 40 minutes. If you find that the crumbles are browning too quickly, cover the cake with aluminum foil and bake for another 20 minutes. Turn off the oven and let cool down slowly with the oven door slightly ajar,