For the short pastry, mix 250 g flour and baking powder. Add 1 packet of vanilla sugar, 25 g sugar, 1 pinch of salt, sour cream and 150 g butter in pieces. Knead first with the dough hooks of the mixer, then quickly with your hands to a smooth dough.
Cover and chill for about 1 hour.
Separate the eggs for the crumbles. Knead the egg yolks with 100 g flour, 40 g sugar, 1 packet vanilla sugar, 1 pinch of salt and 50 g soft butter in a bowl to form coarse crumbles.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash, halve and stone the plums.
Line a baking tray with baking paper. Roll out the dough on a slightly floured work surface (approx. 20 x 40 cm). Roll up the edges all around 3-4 cm and press slightly flat. Whisk the egg white and spread on the edges.
Sprinkle with almonds and sugar crystals. Prick the bottom several times with a fork. Pre-bake in a hot oven on the lowest shelf for 12-15 minutes.
Remove the bottom from the oven. Spread with plum jam and cover with plums. Put crumbles on top. Bake at the same temperature on the middle shelf for 15-20 minutes. Let it cool down and dust with icing sugar.