How to Make It
MAKE THE DOUGH In the bowl of a stand mixer, combine the yeast with 1/4 cup of the warmed milk. Let stand until the yeast is foamy, about 5 minutes. Add the remaining milk, 2 of the eggs, the sugar and salt to the bowl, and using the paddle, mix at low speed until just combined, about 2 minutes. Add the flour and mix until the dough is just moistened, 2 minutes. Switch to the dough hook and mix the dough at low speed, adding the butter a few pieces at a time and mixing well between additions, 6 to 7 minutes total. The dough will be sticky.
Turn the dough out onto a lightly floured surface and kneed until smooth and springy, about 5 minutes. Transfer the dough to a lightly oiled large bowl, turning to coat. Cover the bowl with plastic wrap and let rest in a warm place until doubled in bulk, about 90 minutes.
MAKE THE BLUEBERRY FILLING In a small saucepan, whisk the sugar with the cornstarch and cinnamon. Add the blueberries and orange juice and stir gently until the blueberries are coated in the damp sugar mixture. Cook over low heat, stirring occasionally, until the blueberries begin to burst and the liquid has thickened enough to coat the back of a spoon, about 10 minutes. Remove the saucepan from the heat and let cool completely.
MAKE THE STREUSEL In a small bowl, stir the sugar with the flour and salt. Drizzle in the melted butter and stir until the damp crumbs form.
When the dough is doubled in bulk, lightly dust a rimmed baking sheet with flour. Punch the dough down, transfer to the prepared baking sheet and let rest for 10 minutes.
Meanwhile, line a second rimmed baking sheet with parchment and dust a work surface with flour. Roll the dough out 1/2 inch thick. Using a 2 1/2-inch round cutter, cut out 18 circles. Arrange the circles on the parchment-lined baking sheet about 1/2 inch apart. Coat lightly with nonstick cooking spray and cover lightly with plastic wrap. Let rise until nearly doubled in bulk, about 1 hour.
Preheat the oven to 350°. Remove the plastic wrap. In a small bowl, whisk the remaining egg with 1 teaspoon of water. Using your thumb, make an indentation in the center of each round, leaving a 1/2-inch border. Fill each indentation with about 2 teaspoons of the blueberry filling. Brush the edges of the kolache with the egg wash. Sprinkle each pastry generously with the streusel. Bake for 15 minutes, until the edges are golden brown. Transfer the kolaches to a wire rack and let cool slightly before serving.