Grease a springform pan (26 cm Ø) and dust with flour. Mix flour and baking powder. Add 100 g sugar, 1 pinch of salt, butter in pieces and egg. Knead everything first with the hand mixer (photo), then briefly with your hands to a smooth dough.
Cover and chill for about 30 minutes.
Then 2⁄3 of the dough with some flour and roll out on the springform pan. Close the edge of the mould around it. Form the remaining dough into 2 rolls (each approx. 36 cm long), place them on the edge of the mould (photo) and press them up approx. 4 cm.
Prick the pastry base several times with a fork. Pre-bake in the preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: level 3) for 15-20 minutes.
Clean, wash and cut the rhubarb into pieces of about 2 cm. Mix with 150 g sugar (do not leave to stand for long!). Remove the pre-baked cake base from the oven and immediately spread the rhubarb evenly on top (photo).
Continue baking at the same temperature for about 25 minutes. Remove from the oven and allow to cool.
Soak gelatine in cold water. Stir pudding powder with 50 g sugar and 6 tbsp milk until smooth. Boil up the rest of the milk. Stir in the pudding powder and simmer for about 1 minute while stirring. Remove from heat.
Squeeze the gelatine and dissolve it in the hot pudding while stirring (photo). Let it cool down, stirring several times in between.
Whip the cream until stiff and fold into the pudding in 2 portions. Spread the cream on the rhubarb (photo). Chill the cake for about 4 hours. Remove from the tin and sprinkle with brittle.