Activate the fresh yeast. Melt the dough butter over low heat and let it cool until lukewarm. Stir the crumbled fresh yeast and a small portion of the dough sugar into the lukewarm dough milk, about 90°F (32°C). Rest for 10 minutes. This sugar and milk come from the measured dough ingredients, not extra additions.
Knead the soft dough. Combine the flour, remaining dough sugar, and salt. Add the whole egg, dough yolk, dough lemon zest, cooled melted butter, and yeast milk. Knead for 8–10 minutes until smooth, elastic, and soft, with no heavy sticking. Avoid adding enough work-surface flour to make the dough stiff.
Give the first rise. Cover the bowl and let the dough rise in a comfortably warm place for about 60 minutes, until approximately doubled. Prepare and cool the filling during this rise.
Cook the filling. Heat the filling milk, filling sugar, filling butter, and vanilla sugar in a saucepan, stirring until the butter melts. Stir in the finely ground poppy seeds, soft-wheat semolina, and filling lemon zest. Cook gently over low heat, stirring, for 2–3 minutes until slightly thickened. Transfer to a shallow heatproof bowl and cool completely. It should spread easily without running or releasing a puddle of milk.
Divide and roll. Line a large rimmed baking sheet with parchment rated for the oven temperature. Gently deflate the risen dough and divide into equal roll-size portions, approximately 500 g (about 1 lb 2 oz) each; divide the cooled filling evenly to match. Roll each dough portion on a lightly floured surface into an even rectangle about 12 × 14 inches (30 × 35 cm). Keep the other portions covered. The dough should be thin and even, not paper-thin.
Spread and shape. Spread a filling portion evenly over each rectangle, leaving a ½–¾-inch (1½–2-cm) clean border. Avoid thick mounds and bare gaps. Roll from a long side with enough support to enclose the filling, without stretching the dough tightly. Place seam-side down on the baking sheet with room between rolls.
Proof the shaped rolls. Cover with a clean towel and rise for another 30 minutes. Use more comparable baking sheets when scaling rather than making larger rolls or crowding a tray. During this final rise, preheat a conventional oven to 355°F (180°C), with a rack in the middle.
Brush and vent. Whisk the separate wash yolk and wash milk together. Brush the proofed rolls lightly and evenly, then gently prick the surface at several points with a fork so steam can escape. Discard wash left after contact with raw dough.
Bake through the dough. Bake on the middle rack for about 35–40 minutes, until golden and structurally firm. If browning too quickly, tent loosely with foil for the last part of baking. Check a dough portion near the middle of a roll, not only a pocket of filling: about 190°F (88°C) is a useful secondary bread-doneness guide, alongside no raw or gummy dough. Bake longer if necessary; color alone is insufficient.
Cool before slicing. Rest on the baking sheet for 10 minutes, then transfer carefully to a rack. Allow about 60 more minutes, or longer if needed, until completely cool before cutting. The cooling allowance is an estimate, not a guarantee for every kitchen. Refrigerate promptly within 2 hours, or within 1 hour if the surroundings are above 90°F (32°C); if still warm, finish cooling in the refrigerator before wrapping tightly.
Slice and serve. Cut with a sharp serrated knife without pressing down hard. Each roll gives approximately 12 slices, depending on thickness; the serving unit is one assumed slice, not one whole roll. Serve plain; optional powdered sugar is outside the fixed ingredient and nutrition calculation.