Preheat oven to 180 degrees C. Grease and flour a 23cm ring tin.
In a small bowl, beat the egg whites until stiff peaks form and set aside.
In a large bowl cream the butter with the sugar. Beat in the egg yolks and the lemon zest.
Stir together the flour and baking powder then beat the flour mixture into the creamed mixture. Gently fold in the egg whites. Spread the batter evenly into the prepared tin. Arrange the plums, skin side down, attractively over the batter.
Bake at 180 degrees C for 40 minutes or until a skewer inserted into the cake comes out clean. Transfer to a cooling rack and allow to cool before serving.