Cut butter into cubes. Mix flour, baking powder, salt and 100 g sugar. Knead butter, flour mixture and egg with the dough hooks of the hand mixer. Finally knead with your hands to a smooth dough
Put 2/3 of the dough into a greased and flour-sprinkled springform pan (26 cm Ø) and press down evenly. Form the remaining dough into a roll and press it to the edge. Prick the base several times with a fork. Put the mould in a cool place for about 1/2 hour. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes
In the meantime, clean and wash the rhubarb, cut it into pieces of about 2 cm and mix with 150 g sugar. Remove the pre-baked base and spread the rhubarb evenly on top. Bake at the same temperature for another 20 minutes.
Soak gelatine in cold water. Bring milk to the boil, except for 6 tablespoons. Stir pudding powder, 50 g sugar and 6 tbsp milk until smooth. Stir into the boiling milk, bring to the boil and simmer for 1 minute. Remove from the stove. Squeeze the gelatine and dissolve in the hot pudding while stirring. Put in a cool place and stir from time to time
Whip the cream until stiff and fold into 2 portions under the room-warm, gelatinizing pudding. Pour the cream onto the cooled cake, smooth it down and refrigerate for about 4 hours. Shortly before serving, remove from the tin, sprinkle with brittle and cut into pieces
waiting time approx. 5 1/2 hours