For the base, melt butter and let it cool down a little. Grease an angular springform pan (approx. 35 x 23 x 7 cm) well and dust with flour. Wrap the springform pan in aluminium foil twice up to the edge. Put Shortbread into the freezer bag and close it.
Roll over with a cake roll until the contents are finely crumbled. Knead the crumbs and butter well. Pour the crumb mixture into the tin and press it down to a firm base. Keep cold.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Meanwhile, for the cheese mixture, mix 200 g cream cheese, 150 g sugar, vanilla sugar and 60 g starch on a low setting with the whisks of the mixer for about 3 minutes.
Briefly stir in the rest of the cream cheese, ricotta and 300 g sugar at the highest setting. Stir in the eggs one after the other, then the cream. Carefully spread the cheese cream on the crumb base.
Place the mould in a fat pan (deep baking tray) and pour on so much hot water that the fat pan is almost full to the brim. Bake in the hot oven for approx. 1 1⁄2 hours on the lowest rack until the cheese cream hardly wobbles when you gently shake the pan.
Carefully lift the cake out of the water bath. Place on a cake rack and remove the aluminium foil. Let the cake cool down for about 1 hour. Then cover and leave to cool for approx. 3 hours.
Clean, wash and cut the rhubarb into exactly 7 cm long pieces. Boil the grenadine. Add the rhubarb pieces. Bring to the boil and cook over medium heat for 3-5 minutes, depending on the thickness of the stems. Lift out with a skimmer, drain very well side by side on kitchen paper and let cool off.
Set grenadine aside.
Stir 20-25 g starch and 3 tbsp. water until smooth. Bring the grenadine to the boil again. Add rose water. Stir in the starch. Bring to the boil, simmer for about 1 minute while stirring. Remove from the heat and let it cool down a little while stirring occasionally.
Meanwhile place the rhubarb in rows close together on the cooled cheese mixture. The pieces must finish exactly next to each other and spread the grenadine evenly over the rhubarb. Chill again for about 2 hours.
Carefully remove the cake from the mould. Cut the cake into small pieces so that there are always 2 pieces of rhubarb on each piece. Sprinkle with pistachio sugar as desired.