Preheat oven to 340 °F (170 °C).
Cream the butter with the confectioner’s sugar.
Mix in egg yolks, salt, and lemon zest.
Add coarsely chopped apple and rum.
Beat the egg whites to stiff peaks.
Mix together the nuts, poppy, and flour, and fold nuts and egg whites into the butter and egg yolk mix.
Fold the apple slices into the poppy mixture.
Like a baking sheet with parchment paper, spread the poppy batter on it, and bake for about 60 minutes.
Take the cake out, let cool, and pull off the parchment paper.
Cut up and serve dusted with vanilla confectioner’s sugar.
