Day 1 – Make the dough

1. In the bowl of a stand mixer fitted with a dough hook attachment, combine the all-purpose flour and salt. Mix briefly to combine.
2. In a separate bowl or liquid measuring cup, whisk together the milk, melted butter, sugar, egg, and egg yolk until well combined. Add the sourdough starter to the milk mixture and stir to incorporate.
3. With the mixer on low speed, gradually pour the milk and sourdough mixture into the flour mixture. Mix until a shaggy dough starts to form and no dry flour remains.
4. Increase the mixer speed to medium-low and knead the dough for about 8-10 minutes, until it becomes smooth and elastic. Add in more flour, 1 tablespoon at a time, if needed to form a workable dough. The dough will still be slightly sticky, but will firm up after the overnight cold ferment.
5. Transfer the dough to a clean bowl, cover it with plastic wrap, and let it rise 1 hour at room temperature, then place in the refrigerator overnight.

Make the Cream Cheese Filling

1. Using stand mixer fitted with paddle, or a handheld mixer, beat cream cheese, sugar, flour, vanilla, and salt on low speed until smooth, about 1 minute. Scrape down the sides of the bowl, add 1 teaspoon of lemon juice, and mix to combine. Taste filling – you are looking for the lemon juice to brighten the filling, but not make it taste like lemon. Add more juice if needed. Transfer to bowl, cover with plastic, and refrigerate until ready to use.

Make the Streusel

1. In a small bowl, combine the flour, sugar, and chilled butter for the streusel. Rub the mixture between your fingers until it resembles wet sand. Cover and refrigerate until ready to use.

Day 2 – Shape and Assemble the Kolaches

1. The next day, remove the dough from the refrigerator, punch it down, and turn it out onto a lightly floured surface.
2. Line a rimmed baking sheet with parchment paper.
3. Stretch the dough into an even 9-inch log. Cut the log into 9 equal pieces (about 64 grams, 2 1/4 ounce each) and cover loosely with a kitchen towel.
4. Working one piece of dough at a time (keep remaining pieces covered), form into rough ball by stretching dough around your thumbs and pinching edges together so that the top is smooth. Place the ball seam side down on a clean counter and, using your cupped hand, drag in small circles until dough feels taut and round.
5. Place the dough balls seam side down on a baking sheet lined with parchment paper, leaving enough space between them for rising, about 1 1/2 inches apart.
6. Cover the dough balls loosely with a kitchen towel and let them rise until increased in size by about half, about 2-3 hours.
7. Preheat the oven to 350°F (175°C). Adjust your oven rack for even baking. Grease the bottom of a round 1/3-cup dry measuring cup with butter and lightly dust it with flour.
8. Using the bottom of the measuring cup, press firmly into the center of each dough round to create an indentation for the filling. Re-grease and flour the cup as needed to prevent sticking.
9. Spoon an equal amount of cream cheese filling (about 1 1/2 tablespoons) into each indentation, smoothing the tops with the back of a spoon.
10. In a small bowl, whisk together the egg and water. (I used the leftover egg white.) Gently brush the edges of the dough with the egg mixture.
11. Sprinkle the streusel over the edges of the dough, avoiding the cream cheese filling.

Bake and Serve

1. Bake the kolaches in the preheated oven for 20-25 minutes, or until golden brown. Rotate the baking sheet halfway through baking to ensure even browning. You may need to move the pan up a rack during the final few minutes of baking to achieve a lovely golden color.
2. Transfer the baked kolaches to a wire rack and let them cool for about 20 minutes. Serve warm and enjoy!