Prepare a shortcrust pastry from flour, baking powder, powdered sugar, butter, 1 egg and rum. Chill the smoothly kneaded dough for at least 30 minutes.
Grease the bottom of a springform pan (26 cm) and roll out 2/3 of the dough in it, place 1/3 of the dough as a border (approx. 3.5 cm high).
Pre-bake in the preheated oven at 200 ° C on the middle rack for about 15 minutes (so that the edge does not sink in, blind bake with baking paper and 1 packet of peas).
In the meantime, peel, core and roughly grate the apples. Mix with lemon juice and sugar. Place the grated apple on the dough, which has now cooled down a little, and smooth it out.
Mix the sour cream, cream, egg, custard powder and sugar and carefully pour onto the apples.
Put in the still hot oven and bake at 200 ° C on the lower rail for about 40 - 50 minutes, depending on the type of apple.
Remove the edge of the cake from the springform pan and let the cake cool down in the pan.
Tip: Suitable apple varieties for this are Boskoop or Elstar.