- Line the base and sides of a 23cm pan with a liner. Preheat your oven to 160 degrees C.
- Sprinkle sugar and cardamom on the bottom of the cake pan. Place the cut-side-down plum halves in a circular pattern on top of this sugar.
- Mix butter and sugar with an electric mixer until pale. Beat in the eggs one by one, then add the vanilla and zest. Mix baking powder, almond meal, and polenta. Spread batter evenly over plums and bake for one hour.
- Mascarpone, zest, and juice, along with icing sugar, should be mixed. Turn the cake onto a plate with the plum side up. Mascarpone can be served warm or chilled.