- 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.