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

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

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 dry ingredients to the wet mixture, alternating with the buttermilk until just combined.

In a separate bowl, toss the sliced plums with brown sugar and cinnamon.

Pour the batter into the prepared cake pan and evenly distribute the plum mixture on top.

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

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