Start by preparing the yeast dough in a stand mixer. Put the flour, sugar, egg yolks, grated lemon zest, active yeast into a mixing bowl. Pour in the lukewarm milk.

Knead the ingredients in a stand mixer for two minutes until roughly combined. Then add the softened butter and salt. Knead the dough on medium to high speed (I knead on speed 3-4 out of 5) for 10 minutes.

Using a plastic scraper, remove the dough from the bowl to a floured work surface. Turn the dough once until flour is on both sides.

Pull the dough to the sides and fold it like a letter into an envelope. Repeat twice. Finally, pull the ends of the dough away from the sides and fold them toward the center. Press the ends together to form a ball.

Place the prepared dough ball, seam side down, in a bowl, dust lightly with flour, cover with a clean tea towel, and let rise in a warm place until doubled in volume. This will take about an hour to an hour and a half, depending on the room conditions.

Turn the dough out onto a floured surface and divide it into six equal-sized portions; feel free to weigh them out for equal-sized kolaches if you have a kitchen scale on hand.

Pull the ends out of each piece and fold them over to meet in the middle, then press the ends together. This is a similar process to what you did with the whole dough after you removed it from the bowl.

Place the ball seam side down on a baking sheet lined with parchment paper. This will give the kolaches a nice, even, round shape. Cover with a dish towel and let rise for about 20 minutes.

Before the balls rise, prepare a streusel topping with butter, flour and sugar. Next, wash and dry the plums and remove the pits. If the plums are large, quarter them.

Cover a suitable flat-bottomed container with a clean cloth and dip it in a little flour. Make a well in each of the risen balls. Press down on the center of each first, then shape the edges to complete the well.

Brush the edges of the pressed kolaches with the egg whites left over from making the dough.

Place the plums on the prepared kolaches, cut side up. If the plums are sour, sprinkle them with a teaspoon of sugar. Finally, top with the streusel topping.

Bake the plum kolaches in a preheated oven at 375°F/190°C (upper and lower heating) for 25 minutes until the edges turn golden brown.