Preheat the oven to 370°F/ 190°C. Grease and flour a 9 inches cake pan.
Mix the butter, sugar, milk, vanilla, and eggs. Add the flour and baking powder, and mix until well combined. Do not overbeat.
Pour the mixture into the mold and arrange the plums on top. Bake in the oven for 40-45 minutes.
Sprinkle the cake with sugar and return it to the oven for another 15 minutes or until a knife inserted comes out clean when tested.