Warm about 1/2–3/4 cup of Milk until just lukewarm, then stir in a teaspoon of Sugar and a packet of Instant Dry Yeast; let sit until foamy, about 5–10 minutes.
In a large bowl combine 3–4 cups of All-Purpose Flour with a pinch of Salt and a couple of tablespoons of Sugar. Add the foamy yeast mixture, melted Unsalted Butter, and two Egg Yolks, plus a splash of Vanilla Extract. Mix and knead until you have a smooth, elastic dough (about 8–10 minutes by hand or 5–6 minutes in a mixer).
Place the dough in a lightly oiled bowl, cover, and let rise in a warm place until doubled in size, about 1–1½ hours.
While the dough rises, prepare fillings. For a classic cheese filling, combine Tvaroh with an additional Egg Yolk, a few tablespoons of Sugar, a pinch of salt, a bit of Vanilla Extract and some grated Lemon Zest until smooth.
For a poppy seed filling, toast and grind the Poppy Seeds if whole, then stir them with warm Milk, Sugar and a pinch of Ground Cinnamon and Lemon Zest until thick and spreadable.
For a raisin-cinnamon filling, soak Raisins briefly in warm Milk or a splash of rum, drain, then toss with Sugar and a little Ground Cinnamon. For a simple fruit option, use good-quality Povidla straight from the jar, optionally spiced with a dusting of Ground Cinnamon.
Turn the risen dough out onto a lightly floured surface and divide into even balls (about golf-ball to tennis-ball size, depending on desired koláče). Flatten each ball into a disk, make a shallow well in the center, and spoon in about 1–2 tablespoons of your chosen filling.
Place the filled koláče on a baking sheet lined with parchment, cover loosely, and let rise for about 20–30 minutes until puffy.
Beat one Egg with a splash of Milk and brush the tops of the koláče for a glossy finish. Sprinkle a little extra Sugar or cinnamon-sugar on top if desired.
Bake in a preheated 350°F (175°C) oven for 18–22 minutes, or until golden brown. Cool slightly on a wire rack — these are best served warm.
