Preheat the oven to 375 degrees F (190 degrees C). Grease and flour a 9-inch tube pan.

Separate eggs into whites and yolks. Beat egg whites until stiff peaks form; set aside.

Beat butter and sugar together in a large bowl until light and fluffy. Beat in egg yolks and lemon zest.

Stir flour and baking powder together, then blend into butter mixture. Gently fold in egg whites. Spread cake batter evenly into the prepared pan. There will only be a little over an inch of batter. Arrange plums, skin-side down, in a nice pattern over the batter.

Bake in preheated oven until a tester inserted in the center comes out clean, about 40 minutes. Transfer to a cooling rack and allow to cool before serving.