With the rack in the middle position, preheat the oven to 190 °C (375 °F).
Butter a 33 x 23-cm (13 x 9-inch) baking dish. Add the plums, sugar, and cornstarch and toss to combine.
In a bowl, combine the flour, baking powder, and salt. Set aside.
In another bowl, cream the butter with the sugar and vanilla with an electric mixer. Add the eggs, one at a time, and beat until smooth. At low speed, add the dry ingredients alternately with the milk. Spoon the batter over the plums.
Bake for about 55 minutes or until a toothpick inserted in the centre of the cake comes out clean. Serve warm or cold.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.