Preheat oven to 190°C (375°F), top and bottom heat. Grease a 26 cm (10 inch) springform pan.
In a mixing bowl, beat the margarine or butter with the sugar until creamy. Add the eggs one at a time. Stir in the flour and baking powder to make a smooth batter. Add in the rum and mix briefly.
Fold in the sliced apples. It may feel like there are too many apples, but that’s the beauty of this cake — the batter is just there to hold everything together.
Spread the mixture evenly into the springform pan. It will be uneven on top — that's okay.
Bake on the lower rack for 45–55 minutes, depending on your oven. I have found my electric oven baked quicker vs. my gas oven.
After about 35 minutes, or when the top is no longer doughy, carefully remove the cake from the oven: - Sprinkle with cinnamon sugar - Brush the top with melted butter - Then return it to the oven and finish baking.
Let the cake cool slightly in the pan, then remove the ring and serve warm or at room temperature. I love serving it with fresh whipped cream, but it's also perfect on its own.
Video
Notes
If the top browns too quickly, cover with baking paper during the final part of baking