Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

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

In another bowl, cream the softened butter and granulated sugar until light and fluffy.

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

Gradually add the flour mixture to the wet ingredients, alternating with the milk, and mix until just combined.

Pour the batter into the prepared cake pan and smooth the top.

Arrange the sliced plums on top of the batter and sprinkle with brown sugar.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.