Melt butter for the bottom. Put Shortbread in a freezer bag, close and crumble finely with a cake roll. Mix crumbs and butter. Wrap the bottom edge of a springform pan (26 cm Ø) from the outside with aluminium foil so that the pan is absolutely tight. Put the crumbs into the tin and press them down to a flat base. Chill for about 30 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Mix quark, sugar, vanilla sugar and starch with the whisks of the mixer. Stir in cream and eggs. Slide a fat pan (deep baking tray; approx. 32 x 39 cm) into the hot oven on the middle shelf. Fill it up to about 1 cm high with boiling water. Fill the curd mixture on the shortbread crumb base into the pan and smooth it down. Place the pan in the fat pan and bake for about 10 minutes.
Turn down the oven temperature (electric cooker: 125 °C/circulating air: 100 °C/gas: see manufacturer). Continue baking for approx. 1 hour. Remove from oven and let cool on a cake rack. Carefully remove from the pan.
Mix the smoothie and jam sugar according to the package instructions using a hand blender or whisk of the mixer for approx. 45 seconds. Spread immediately on the cheesecake. Keep cool until serving.