Make ahead: Before you start making the yeast dough, take the egg and butter out of the fridge about an hour in advance to bring them to room temperature.
In ½ cup lukewarm milk, whisk 1 Tablespoon all-purpose flour, 1 teaspoon granulated sugar, and 1 ½ teaspoon active dry yeast. Leave in a warm place to activate until bubbles appear on the surface. This will take about 10–15 minutes.
Pour 2 ½ cups all-purpose flour into a mixing bowl. Add the activated yeast mixture, the ½ cup lukewarm milk, ⅛ teaspoon salt, and 1 egg yolk.
Knead in a mixer with a dough hook attachment on medium speed for 10 minutes, until the dough is smooth and elastic.
Transfer the yeast dough to a bowl and cover with plastic wrap. Leave in a warm place to rise for 45 minutes. The dough should double in volume during this time.
Divide the proofed dough into equal pieces and roll each one into a ball about the size of a baseball. Cover with a clean cloth and let them proof a second time for about ten minutes.
Heat a frying pan over medium-high heat without adding any oil. Carefully stretch the raised dough balls into pancakes roughly the size of your palm.
Fry the Beleše pancakes on both sides for about 2 minutes each. Repeat until all the pancakes are cooked.
Melt 3 Tablespoons unsalted butter in a saucepan on the stove. Brush the hot pancakes with butter on one side. Then, brush the pancakes with plum butter on the same side you have buttered them. Use about two teaspoons of plum butter per pancake.
Combine ⅓ cup ground poppy seeds and ½ cup powdered sugar in a shallow bowl. Press each pancake, plum butter side down, into the poppy seed mixture.