Preheat the oven (top/bottom heat: approx. 200 degrees, gas: level 3 – 4). Grease a loaf pan (25 x 11 cm) or line with baking paper.
Cut the prunes into small pieces. Put all ingredients – except the prunes – into a mixing bowl and mix well with a hand mixer (whisk). Fold in the plums and pour the batter into the mold. Place the pan on the rack in the lower third of the oven for about 50 minutes.
Let the cake sit in the turned off oven for about 30 more minutes. Then place the cake on a cooling rack and let it cool in the pan.