Mix the butter (not too cold) with sugar, lemon zest and vanilla sugar until smooth. Add the egg and milk, then mix in the flour and baking powder and knead into a dough. Let rest for about 30 minutes.
Peel and core the apples, slice thinly and mix with sugar, breadcrumbs, raisins, pine nuts, rum, vanilla sugar, cinnamon and lemon zest.
Preheat the oven. Roll out the dough on a floured surface and place on a buttered baking tray. Spread the apple filling over the dough and fold into a strudel. Brush with beaten egg and bake.
Dust with powdered sugar and serve.
Temperature: 180°C
Baking time: 35 minutes