Mohnstollen (Poppy Seed Stollen)

Place flour into a large bowl and make a well in the center. Crumble fresh yeast into the well, then pour in lukewarm milk. Mix yeast and milk together in the well to combine, leaving flour untouched. Cover and let sit in a warm place until foamy, about 15 minutes.

Mix a little flour at a time into yeast mixture until no flour remains. Add butter, sugar, and salt; knead until a soft, pliable dough forms. Cover with a clean dish towel and let rise in a warm place until doubled in volume, about 1 hour.

Grease a baking sheet with butter. Roll dough on a lightly floured surface into a thin rectangle.

Make the filling: Place ground poppy seeds into a bowl. Pour hot milk over top, then add sugar, raisins, honey, vanilla sugar, lemon zest, and rum. Stir until well combined.

Spread filling over dough. Start at a longer edge, and roll dough up and over filling into a log; press the seam together with your fingers to seal. Place the log, seam-side down, onto the prepared baking sheet. Cover and let rise for 20 minutes.

Meanwhile, preheat the oven to 350 degrees F (175 degrees C).

Bake in the preheated oven until golden brown, about 40 minutes. Allow to cool for about 1 hour, then dust with confectioners' sugar.