For the crumble, cut the butter into small pieces, mix with the flour and sugar and rub into coarse crumbs. Place the streusel on a plate and chill.
Grease a baking pan with butter and sprinkle with flour. Preheat the oven to 180 ° C.
Halve and core the plums. Sieve the flour with baking powder. Mix the butter, sugar, vanilla sugar and lemon zest until creamy. Gradually add the eggs and stir until frothy. Stir in the liqueur, fold in the flour. Spread the mixture into the mold and cover with the plums (cut surface above). Cover the cake with crumble and bake in the oven (middle rack) for about 50 minutes.