Preheat your oven to 350°F (175°C).

Grease a 9-inch round cake pan with butter or non-stick spray.

In a mixing bowl, beat the softened butter and granulated sugar together until light and fluffy.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry mixture to the wet ingredients, mixing until just combined.

Fold in the sliced plums gently.

Pour the batter into the prepared cake pan, smooth the top, and sprinkle brown sugar over it.

Bake for 40-45 minutes, or until a toothpick comes out clean.