Preheat your oven to 350°F (175°C).
In a mixing bowl, cream together the sugar and butter until light and fluffy.
Add the eggs one at a time, beating well after each addition.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Stir in the vanilla extract.
Fold in the sliced plums.
Pour the batter into a greased and floured cake pan.
Bake for 50 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.