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The most common cause is milk that was too hot when the yeast was dissolved in it; the recipe calls for lukewarm milk for exactly that reason, since heat above body temperature kills the yeast. A cold kitchen also slows things down, so let the dough rest somewhere warm and draft-free. Also check that your dry yeast is still within date, as an egg-and-butter dough like this one is heavy and needs active yeast to lift it.

Bread flour works well here and gives the twisted strands a bit more structure, which helps them hold their shape in the loaf pan. If you use whole-grain flour, replace only part of the 250 g, because the bran cuts the swirl and the dough absorbs more liquid - you may need a splash more milk than the 110 ml listed. Keep the sugar, salt, and butter amounts as written so the crumb stays soft.

The filling contains 60 g of butter, which melts and runs if the butter was too soft or close to melting when you mixed it - it should be soft but still creamy, and the 1 tsp of flour is there to help bind it. Spreading it too thickly near the edges also pushes it out when you roll the log up tightly. Some butter escaping into the pan is normal and just gives you a caramelized bottom crust.

Yes - after kneading, cover the dough and let it rise slowly in the fridge overnight, then bring it to room temperature before rolling it out. You can also shape the twist, place it in the loaf pan, and refrigerate it covered; in the morning let it sit out until puffy instead of the 30 minutes at room temperature. Brush with the egg yolk only right before it goes into the oven at 180 °C (350 °F).

Let it cool completely as the recipe says, then keep it wrapped or in an airtight container at room temperature - refrigerating dries out this kind of enriched dough. It freezes well whole or in slices; wrap tightly and thaw at room temperature, then warm briefly in the oven to bring back the soft crumb. Dust with the powdered sugar only after warming, since it dissolves on a warm surface.