Crumble the yeast and mix with the lukewarm milk, roughly 1 tablespoon of the sugar, and a small handful of flour. Let rise in a warm place (around 25-27°C / 77-80°F) for roughly 10 minutes until foamy. Add the remaining flour, sugar, butter, egg yolks, vanilla sugar, salt, rum, and lemon zest, and mix until smooth. Cover and let rise in a warm place for roughly 1 to 1.5 hours until the dough has doubled in size.
Toast the breadcrumbs in butter over medium heat for roughly 3 to 4 minutes until golden and fragrant. While toasting, peel the apples and cut them into thin slices.
Roll out the dough to roughly 1 cm thick on a floured work surface. Spread the toasted breadcrumbs evenly over the dough. Layer the apple slices on top and sprinkle with the remaining sugar, a pinch of cinnamon, and a handful of raisins. Roll up tightly into a strudel shape, place on a baking sheet lined with baking paper, and let rise in a warm place for roughly 20 to 30 minutes.
Bake at 180°C (356°F) for about 40 minutes. When the surface turns light golden brown (around 20 to 25 minutes into baking), brush with melted butter and continue baking until done.