Place flour in a mixing bowl, make a dimple in the middle. Put crumbled yeast there, sprinkle with 1 teaspoon of sugar, pour over ¼ cup of lukewarm milk. Mix with a fork until a runny batter is created in the middle. Cover the bowl with plastic wrap and let rise in the warm place for 30 minutes.
Meantime, melt butter over low heat, mix it with egg yolks, rest of lukewarm milk, vanilla essence, lemon zest, and salt. Whisk until combine.
Add whisked butter-egg liquid to the bowl with flour and activated yeast. Add sugar.
Knead a smooth dough: Start mixing with a fork, then use your hands to make a nice dough. It takes about 5-8 minutes. Or use a kitchen mixer with proper attachment if you have one. Let the dough rise for 45 minutes.
Meantime, make cream cheese (tvaroh, quark) filling. Get plum jam ready.
Make streusel topping: mix flour, butter, and sugar using your fingers to create small coarse crumbles.
When the dough has risen, dump it on the worktop. The dough should not stick, if it does, flour the worktop a little.
Roll the dough out into a square thick about ¼ inch – the rolled dough can’t be too thin! Divide into 16 equal pieces.
Place 1-2 teaspoons of cream cheese filling in the middle, then fold the opposite sides and seal together carefully. Shape the filled dough pieces between your palms into a ball.
Transfer the kolache to a baking sheet lined with parchment paper. Place each kolach with the seamed side down. Let the kolache rise for another 30 minutes.
Preheat oven to 340 °F (170 °C).
For an egg wash, whisk one egg in a bowl using a fork. Press each kolach in the middle, for example, with a joint of your index finger.
Brush kolache with egg wash and fill holes with plum jam. Sprinkle with streusel topping.
Bake kolache for 20-25 minutes until golden brown.