In a small saucepan, bring the sugar and water to a boil. Add the plums and remove from the heat. Let rest for 20 minutes. Drain.
With the rack in the middle position, preheat the oven to 350°F (180°C). Butter the sides of an 8-inch (20 cm) springform pan and line the bottom with parchment paper.
In a bowl, combine 2 cups (260 g) of the almond powder, the flour and baking powder. Set aside.
In another bowl, cream the butter and sugar with an electric mixer. Add the eggs, one at a time, beating until smooth. At low speed or with a wooden spoon, stir in the dry ingredients. Set aside ½ cup (125 ml) of the batter. Spoon the remaining batter into the prepared pan and place the plums, cut side down, on the surface.
Stir the remaining almond powder (1/2 cup) into the reserved batter. Spoon into a pastry bag fitted with a pastry tip. Pipe the mixture in the spaces between the plums and sprinkle with the almond slices. Place the pan on a baking sheet.
Bake for 1 hour or until a toothpick inserted into the centre of the cake comes out clean. Let cool completely in the pan.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.