Make the sponge: add a spoonful of sugar to the warm milk and stir in the yeast. Leave for 15-20 minutes, until the sponge is bubbly and has risen into a cap.

In a separate bowl, cream the sugar with the margarine using a whisk, add the egg, and mix. Pour in the sponge, add the flour and salt, and knead the dough.

Leave the dough in a warm place for 2 hours to rise well.

While the dough rises, make the poppy seed filling: heat the milk with the sugar in a saucepan, add the poppy seeds, and simmer for 7-10 minutes. Turn off the heat and add a piece of butter. Blend the filling so the poppy seeds aren't crunchy.

Punch down the risen dough and roll it out into a rectangular sheet. Spread the filling over it evenly.

Roll it up, pinch the edges well, and roll it firmly. Brush with beaten egg yolk and sprinkle with poppy seeds if desired. Let rest for 30 minutes to proof.

Bake in an oven preheated to 190-200°C for 25-30 minutes.

Let the finished roll cool. Enjoy!