Melt the butter and add the sugar. Beat them with a mixer to a smooth, homogeneous mixture. Add the eggs, sugar and cream and continue beating.
Sift the flour and add it together with the baking powder and vanilla sugar to the liquid mixture. Stir to obtain a thick dough.
Smear a suitable tray generously with butter and pour the mixture in it. Arrange the cleaned, pitted prunes on top after rolling them in starch.
Bake in a preheated 356°F (180 °C) oven to a brown crisp. Check with a wooden skewer to see if it's ready.
Put the ready cake in a suitable platter and sprinkle with powdered sugar. Cut it up once cooled.