Preheat the oven: Preheat the oven to 350°F (175°C). Grease and flour an 8- or 9-inch round cake pan.
Preparing the plums: Mix the plum slices with lemon juice and set aside.
Mixing the dry ingredients: In a medium-sized bowl, mix the flour with the baking powder and salt.
Buttercream and Sugar: In a large bowl, cream the butter, granulated sugar and brown sugar until light and fluffy.
Add the eggs and vanilla: Add the eggs one at a time, then add the vanilla extract.
Combine: Gradually add the dry ingredients to the wet mixture, alternating with the milk, starting and ending with the dry ingredients. Mix until just combined.
Arrange the plums: Pour the mixture into the prepared pan. Arrange the plum slices on top of the dough in a circular shape.
