Yeast dough: First, activate the yeast. Pour ⅓ cup lukewarm milk into a cup. Stir in 1 Tablespoon all-purpose flour, ½ teaspoon granulated sugar, and 2 ½ teaspoons active dry yeast. Let it sit in a warm place for about 10–15 minutes until foam with bubbles appears on the surface.
Pour 4 cups all-purpose flour into a mixing bowl. Add 2 yolk, 1 cup lukewarm milk, ⅓ cup granulated sugar, and activated yeast.
Place the mixing bowl in the kitchen stand mixer fitted with the dough hook and knead on medium speed for about two minutes. Turn off the mixer and add 1 stick unsalted butter (softened), cut into small pieces, and ¼ teaspoon salt to the bowl. Turn the mixer back on and knead at medium speed for 8 minutes until smooth and elastic.
Remove the dough from the hook, shape it into a ball, and place it in a sufficiently large, clean bowl. Cover it with plastic wrap and let it rise in a warm place until the dough doubles in size. Depending on the surrounding conditions, this usually takes about one to two hours.
Farmers cheese filling: Meanwhile, prepare the kolache filling with the farmer's cheese (Czech Tvaroh cheese). Place 18 ounces farmers cheese in a bowl with 2 yolks, ⅔ cup granulated sugar, ½ teaspoon vanilla extract, and 1 ½ Tablespoons cornstarch. If you like, add ⅓ cup raisins. Is the farmer's cheese too dry, add a tablespoon or two of milk. Stir until combined. The kolache filling should have a firmer, paste-like consistency.
Crumbly topping: Next, prepare the crumble topping (Czech Drobenka or Posypka). Put 3 Tablespoons all-purpose flour, ⅓ stick unsalted butter into pieces, and 2 Tablespoons granulated sugar into a bowl. Work with your fingers until you get a crumbly mass.
Finishing: Turn the proofed dough out onto a lightly floured work surface. Briefly knead the dough by hand, then divide into two equal halves. Roll each half into a square or rectangle. Cut the rolled-out dough into squares about 3 inches (8 cm) on each side.
Place a tablespoon of the farmer's cheese filling in the middle of each square. Fold the opposite ends of the dough over the filling and press them together. Try to make sure the ends are folded over each other enough. Press well to seal. If you underestimate this step, the kolache may untied during proofing or later when baked in the oven.
Line a baking sheet with parchment paper and carefully transfer the kolache to it. Leave a space of at least 1 1⁄2 inches (4 cm) around the kolache as they will puff up during baking.
Cover the kolache with a clean tea towel and let them rise in a warm place for another 30 minutes. If the tied kolache come apart, carefully try to bring the ends back together and press firmly.
Use your finger to gently press down on the seam and make a shallow indentation in the center of each kolach. Now, brush the kolache with the remaining egg whites from preparing the yeast dough. Finally, sprinkle the crumble topping over the indentation.
Preheat the oven to
350 °F. Bake for about 17 minutes on a rack in the center of the oven. The kolache will be golden brown on top. Once baked, transfer the baking sheet to a cooling rack and allow the kolache to cool.