01 - Preheat the oven to 180°C (350°F). Grease a cake pan or line it with parchment paper.
02 - In a mixing bowl, cream the softened butter and sugar until light and fluffy.
03 - Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
04 - In a separate bowl, combine the flour, baking powder, cinnamon, and salt. Gradually add this to the butter mixture, mixing until just combined.
05 - Fold in the marzipan pieces gently into the batter.
06 - Pour half of the batter into the prepared cake pan. Arrange the halved plums cut-side up on top of the batter, then cover with the remaining batter.
07 - Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
08 - Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.