Preheat your oven to 350°F (175°C) and grease a round cake 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.
Stir in the vanilla extract and lemon zest, if using.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing just until combined.
Gently fold in the sliced plums until evenly distributed.
Pour the batter into the prepared cake pan and smooth the top.
Bake 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, then transfer to a wire rack to cool completely.