- With the rack in the middle position, preheat the oven to 180 °C (350 °F). Butter and line a 20 cm (8-inch) springform pan with parchment paper. Place on a baking sheet.
- In a bowl, combine the flour, baking powder and baking soda. Set aside.
- In another bowl, cream the butter and sugar with an electric mixer. Add the egg and egg whites, one at a time, and beat until the batter is smooth. At low speed, add the dry ingredients alternately with the buttermilk. Set aside.
- In a skillet, melt the butter. Add the sugar and bring to a boil. Cook for 1 minute. Add the plums and toss well. Spread at the bottom of the prepared pan. Cover with the batter.
- Bake for about 55 minutes or until a toothpick inserted in the centre of the cake comes out clean. Let cool for 15 minutes on a wire rack. Flip onto a serving plate and let rest for 15 minutes before unmolding. Will keep for a few days under a glass dome.