1. Wash, stone, and quarter the plums. Place in a non-stick pan with the sugar, vanilla sugar, cinnamon, and water, and heat up. Let the plums simmer over medium heat until they have softened slightly and the sugar has caramelized. Remove from heat and let cool down completely. You can do that the day before and store the cooled plums in the fridge until needed.

2. Preheat the oven to 350°F (180°C). Line a 9.5 inches (24cm) springform tin with baking parchment and grease lightly. Set aside.

3. Add margarine and sugar to a large bowl and mix on high speed until light and fluffy. Add the eggs one after another and mix well after each addition. Add vanilla extract and mix in. Mix the flour with cocoa powder, baking powder, speculoos spice, and salt, then add together with the milk to the bowl and mix on low until just combined. Add the chopped chocolate and fold in. Transfer the batter to the prepared springform tin, smooth out the top and bake for about 24-26 minutes or until a toothpick inserted into the center of the cake comes out clean. Take out of the oven and let cool down completely on a wire rack.

4. Add the cold heavy cream, vanilla extract, and ground cinnamon to a bowl and mix until frothy. Add the cream stiffener and whisk until stiff peaks form. Add the chopped chocolate and crumbled cookies and fold in.

5. Cut off the (presumably) domed top of the cake. You need an even cake layer that has a thickness of at least 0.4 inches (1cm). Crumble the rest of the cake to get fine crumbs. Spread the cinnamon plums evenly on top of the cake layer, leaving the edge of the cake free. Add the whipped cream on top of the plums, completely enclosing the fruits with it and shaping the cream into a round dome. Sprinkle the cake crumbs all over the cream and press them lightly into the cream. Decorate with some (star) cookies if you like. Place in the fridge for at least 30 minutes before serving.