Prepare yeast dough: First, proof the yeast. Pour a teaspoon of sugar into ½ cup of warm milk, add 1 ½ teaspoons active dry yeast, and stir. Let the yeast activate in a warm place; that means the yeast should be foamy on the surface. It all takes 10–15 minutes.
In the mixing bowl, pour in 1 ¾ cup all-purpose flour, remaining sugar and warm milk, 1 egg yolk, and the activated yeast mixture. Start the kitchen stand mixer at medium speed for about a minute to roughly combine the ingredients. Then add ⅓ stick unsalted butter and ⅛ teaspoon salt, continuing to knead the dough—for about ten minutes—to achieve a very smooth and elastic texture.
Remove the finished dough from the dough hook, shape it into a ball, and place it in a bowl. Cover it with a clean kitchen towel or plastic wrap, and let it rise in a warm spot until it doubles in size. This usually takes about an hour to two, depending on the surrounding conditions.
Make poppyseed filling: While the dough is rising, prepare the poppy seed filling. In a saucepan, heat ⅔ cup milk with ½ cup granulated sugar; add 1 ½ cup ground poppy seeds, ⅓ cup raisins, ¼ teaspoon ground cinnamon, 2 teaspoons lemon zest, and 1 teaspoon vanilla paste. Stir everything together. Reduce the heat and simmer for about ten minutes. Finally, add 1 Teaspoon spiced rum. Let the filling cool on the kitchen counter.
Turn out the raised dough onto a lightly floured surface. Roll the dough into a roughly rectangular shape, if possible. The size depends on the baking sheet—the roll should comfortably fit lengthwise on the sheet.
Spread the chilled poppyseed filling onto the rolled-out dough, leaving about an inch of space along the edges. Fold the shorter sides over the filling first, then roll the longer side towards the other edge.
Line a baking sheet with parchment paper. Place the rolled strudel on it seam side down and let it rise in a warm place for another half an hour (the so-called second rise).
Brush the raised roll with 1 egg white you reserved earlier when preparing the dough. Use a fork to prick the surface of the roll to prevent it from cracking.
Preheat the oven to
350 °F. Bake the roll for 30 minutes until the surface turns golden brown.