Stone the plums and divide into contiguous quarters. Put aside.
Beat the butter until foamy. Add sugar to the butter and stir until the mixture turns whitish. Gradually add the eggs and continue to stir vigorously until everything is lightly frothy. Mix the flour, baking powder and vanilla sugar and stir in briefly. Stir in the amaretto. Stir in enough milk to make a smooth dough.
Spread the dough evenly on the baking sheet greased with a little butter.
Cover the dough with the plums and bake for about 30 minutes at 175 ° C top / bottom heat on the middle rack. The cake is ready when the batter is golden brown between the plums. Immediately sprinkle the cake with the cinnamon sugar.