Preheat your oven to 350°F (175°C) and grease a bundt pan.
In a large bowl, cream together the softened butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Mix in the vanilla extract.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with milk.
Stir until just combined; do not overmix.
Fold in the sliced plums gently.
Pour the batter into the prepared bundt pan and smooth the top.
Bake for 40-45 minutes, or until a toothpick inserted comes out clean.
Allow the cake to cool in the pan for 10 minutes before inverting onto a wire rack.