Preheat your oven to 350°F (175°C). Grease a round cake pan and line it with parchment paper.

Halve and pit the ripe plums. Arrange them cut-side down in the bottom of the prepared pan, nestling them close together.

In a small saucepan, melt butter and brown sugar over medium heat, stirring until bubbly. Pour this mixture over the arranged plums.

In a separate bowl, cream together granulated sugar and butter until light and fluffy. Add eggs one at a time, mixing well.

In another bowl, sift together flour, baking powder, and any spices like cinnamon or nutmeg.

Alternate adding the dry mixture and milk to the creamed mixture, mixing until just combined.

Carefully pour the batter over the plums and smooth it out with a spatula.

Bake in the preheated oven for about 40-45 minutes or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes before placing a plate over the top and flipping it to reveal the plums.