Preheat oven to 190 C / Gas 5. Grease and flour one 23cm tube cake tin.
Separate the eggs. In a small bowl, beat the egg whites until stiff peaks form; 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 and then beat the flour mixture into the creamed mixture. Gently fold in the egg whites. Spread the batter evenly into the prepared tin. There will only be a little over 2.5cm of batter. Arrange the plums, skin side down, attractively over the batter.
Bake in preheated oven for 40 minutes or until cake tests done. Transfer to a cooling rack and allow to cool before serving.