First peel and cut apples. Line the bottom of a springform pan with parchment paper, as well as attach the edge of the springform pan with parchment paper. Brush the paper thickly with 30 g butter (use a brush or finger), sprinkle with sugar and top with the walnuts. Then add the apple pieces loosely.
For the dough, stir the butter until foamy. Gradually add the sugar and eggs. Mix flour and baking powder, sieve and stir in together with the rum. If necessary, add the grated lemon zest. Spread the batter over the apples.
Bake in a preheated oven at 175 ° C (top / bottom heat: 190 ° C) for approx. 45-50 minutes.
Remove and let cool in the mold for about 10 minutes. Loosen the edge. Turn the cake out onto a cake plate and peel off the baking paper. Let cool down