01 - Preheat oven to 350°F (175°C). Grease a 9-inch springform pan and line the base with parchment paper.
02 - In a large bowl, beat softened butter, grated marzipan, and sugar together until pale and fluffy.
03 - Add eggs one at a time, mixing thoroughly after each addition. Blend in vanilla extract.
04 - Whisk flour, baking powder, salt, and ground almonds in a separate bowl.
05 - Fold the dry ingredients into the marzipan mixture gently until just blended.
06 - Spread batter evenly in prepared pan. Arrange plum halves, cut side up, over the surface, pressing lightly.
07 - Sprinkle with sliced almonds and coarse sugar, if desired.
08 - Bake for 40 to 45 minutes until golden and a skewer inserted in the center comes out clean, allowing for some plum moisture.
09 - Let cake cool in the pan for 15 minutes. Remove pan ring and allow to cool completely on a wire rack.