Carefully measure the flour and whisk it with the salt in the mixer bowl. (The best way to measure it is to use a food scale or gently place flour in a measuring cup and level the top with a knife.) Set aside.
Add the sugar to the melted butter in a separate large bowl with a handle. Add the yeast when the mixture is 100-110°F. Let it sit for about 10 minutes until the yeast foams.
Mix the eggs with the milk in a small handled measuring cup and add to the butter mixture.
Turn the mixer on low and slowly pour the butter/egg mixture into the flour, letting it mix for 2 minutes or until all the ingredients are wet. Scrape down the sides of the bowl.
Turn the mixer to medium-low and knead for 10 minutes. The dough will become smooth and elastic and eventually pull away from the sides of the bowl.
Lightly grease a large bowl. Form the kneaded dough into a ball. Then, place it seam-side down in the bowl. Cover with plastic wrap and place it in a warm place to rise until doubled in size, 1 1/2 to 2 hours.
Line 2 large baking sheets with parchment paper. Punch the dough down to deflate it. Place it on a floured surface and roll it into a 16-inch log. Slice the log into 16 equal pieces.
Keep the dough covered with plastic wrap while working. Roll each piece into a ball with a smooth top side. Place the balls seam-side down onto the baking sheets, spacing them 1 1/2 inches apart.
Place in a warm place and let rise for 1 1/2 to 2 hours. The balls will flatten as they rise.
Preheat the oven to 350°F. Position the oven racks in the center of the oven. Firmly press a 1/3 measuring cup down in the center of each dough ball to make an indentation. Add filling to the indentation of each kolache. Brush the outer rims with the egg wash and sprinkle streusel on the rims.
Bake for 25 minutes, rotating and switching the pans after 12 minutes. Cool for a few minutes after removing the kolaches from the oven. Serve warm.