Soak the Raisins: Add the raisins and rum to a small container, mix well, and leave them covered or resting on the counter overnight so the raisins can absorb the rum.
Make the Dough: Combine the flour, sugar, and baking powder in a large bowl. Add the cold butter pieces and the egg, then mix and knead until a smooth dough forms.
Chill the Dough: Wrap the dough tightly in plastic wrap and refrigerate it for at least 1 hour.
Prepare the Custard Mixture: In a small bowl, combine the vanilla custard powder, sugar, vanilla extract, and a small amount of the milk, mixing until smooth.
Heat the Milk and Cream: Pour the remaining milk and the heavy cream into a saucepan and bring the mixture to a boil.
Cook the Custard: As soon as bubbles begin rising through the milk mixture, add the custard powder mixture and stir continuously. Cook for 1 minute while stirring constantly. Cool the Custard: Remove the saucepan from the heat and allow the custard to cool for a few minutes.
Add the Eggs: Stir the lightly beaten eggs into the slightly cooled custard until incorporated. Set the custard aside and stir it occasionally to help prevent a skin from forming. Preheat the Oven: Preheat the oven to 180°C (350°F).
Prepare the Springform Tin: Line a 26 cm springform tin with parchment paper.
Roll Out the Dough: Place the chilled dough on a lightly floured surface and roll it into a circle slightly larger than the springform tin.
Form the Cake Crust: Transfer the rolled dough to the prepared tin, then press it across the bottom and up the sides to create the cake crust.
Chill the Crust: Place the prepared springform tin in the refrigerator until you are ready to assemble the cake.
Prepare the Apples: Peel the apples and remove their cores using an apple corer, keeping the apples whole.
Arrange the Apples: Place the prepared apples closely together inside the cake tin, packing them as tightly as possible so they remain in position when the custard is added.
Fill the Apples: Spoon or place the rum-soaked raisins into the hollow centers of the apples.
Add the Custard: Pour the slightly cooled custard evenly over the apples, allowing it to settle around them.
Bake the Cake: Place the cake in the preheated oven and bake for about 70–80 minutes, or until the custard filling has set nicely.
Add the Almonds: About 15 minutes before the end of the baking time, sprinkle the sliced almonds over the top of the cake, then return it to the oven to finish baking.
Protect the Apples: If the apples become too dark during baking, loosely cover the cake with aluminum foil or baking paper and continue baking.
Cool Completely: Remove the finished cake from the oven and allow it to cool completely on a wire rack while still inside the springform tin.
Dust Before Serving: Once completely cooled, sprinkle the cake lightly with confectioners’ sugar if desired, then serve.
