This is a classic breakfast favorite in Texas. You can save a lot of rising time by preparing the dough the evening before and keeping it in the refrigerator (covered). The next morning, you'll have only the second rise to wait out before baking these blueberry-filled breakfast pastries.
Line 2 baking sheets with parchment paper; set aside.
FOR THE DOUGH: Combine milk, butter and eggs together in measuring cup (will be lumpy).
Combine dry ingredients in the bowl of a standing mixer, then add milk mixture and mix for 2 minutes on low speed. Increase speed to medium and continue mixing for 8 to 12 minutes until dough no longer sticks to side of mixing bowl.
Place dough in a lightly oiled bowl and let rise in warm place for 1 hour, or refrigerate overnight in a covered bowl.
FOR THE FILLING: Combine frozen berries, sugar and cornstarch in a microwave safe bowl and mix well. Microwave on high for about 6 to 8 minutes and stir halfway through cooking.
FOR THE CRUMB TOPPING (if using): Combine the flour, sugar and butter with your fingers until it forms coarse crumbs. Cover and refrigerate until needed.
TO PREPARE THE KOLACHES: Divide dough into 16 equal size portions and form balls. Place dough balls on prepared baking pans, cover with plastic and allow to rest in warm place for 1.5 hours.
Preheat oven to 350°F.
Grease and flour the bottom of a 1/3 cup measuring cup
Place the greased measuring cup on top of each dough ball, and press down to make an indentation in each dough ball. Fill each indentation with about 1 1/2 tablespoons of the blueberry filling.
Mix together the egg and milk, and brush top of each of the Kolaches with the egg wash.
Bake for 20 to 25 minutes (do not over bake - they should be a little pale in color), and let cool for about 10 to 15 minutes. Serve warm.