Make a yeast starter: Heat 1 cup lukewarm milk to about 110 °F. Stir in one tablespoon of sugar and ½ Tablespoon active dry yeast. Leave in a warm place. In about 10–15 minutes, light brown foam with bubbles should form on the milk surface.
Start making a yeast dough: Heat ½ stick unsalted butter in the microwave for 30 seconds or melt it on the stovetop. The butter cannot be hot! In a large mixing bowl, add 4 cups all-purpose flour, melted butter, 2 yolks, ½ teaspoon vanilla paste, ¼ teaspoon salt, the rest of 4 Tablespoon granulated sugar, and the yeast starter.
Knead the dough smooth. First, use a wooden spoon to mix the dry and wet ingredients in a bowl. Once the dough becomes crumbly, transfer it to a floured work surface. Knead by hand until it forms a soft, slightly sticky dough. If it sticks too much, sprinkle with flour and continue kneading. The entire kneading process takes about 10 minutes and requires some physical effort if done by hand.
Let the dough rise for an hour in a warm place or until it doubled in size.
In the meantime, make poppyseed filling. Pour 1 cup milk into a saucepan and add7 ounces ground poppy seed, 4 Tablespoons granulated sugar, ½ teaspoon vanilla paste, 1 Tablespoon lemon zest (grated), and 3 cloves (ground). Bring to a boil, then reduce the heat to low and simmer for 15 minutes, stirring occasionally. Let the poppy seed filling cool completely.
Roll out the dough into a 30x12-inch rectangle (50x30cm). The dough should not be overly thin! Spread the cooled poppy seed filling evenly over the surface of the dough, leaving about 1 inch (2.5 cm) around the edge free.
Fold the dough over the filling from both sides. Basically, you create a simple roll.
Prepare your bundt cake pan. Carefully grease the inside with 1 Tablespoon solid fat (e.g. Crisco) and dust with fine, taste-neutral 2 Tablespoons breadcrumbs or flour.
Place the filled, folded dough into the prepared bundt pan, ensuring it fills the pan evenly. Let it sit for the second rise in a warm place for 45 minutes.
Preheat the oven to
350 °F and set an oven rack in the middle position. Bake the cake for 1 and ¼ hours. If it starts to turn golden brown on the surface near the end, cover loosely with foil.