Start by preparing the yeast dough in a stand mixer. Put 3 cups all-purpose flour, ⅓ cup granulated sugar, 2 egg yolks, ½ Tablespoon lemon zest, and 2 teaspoons instant yeast into a mixing bowl. Pour in ⅔ cup milk (lukewarm).
Knead the ingredients in a stand mixer for two minutes until roughly combined. Then add ½ stick unsalted butter (softened) and ¼ teaspoon 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.
Dump the dough on a floured surface and divide it into six equal portions. If you have a kitchen scale, feel free to weigh each portion for perfectly sized kolaches.
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 ⅓ stick unsalted butter, ⅓ cup all-purpose flour and ¼ cup granulated 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 1 egg white left over from making the dough.
Place 2 pounds fresh plums (pitted and halved) 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
380 °F for 16 minutes until the edges turn golden brown.