Add the lukewarm milk, sugar, cooled butter, eggs, and yeast to the bowl of a stand mixer. Mix everything together using the paddle attachment.
1 cup whole milk, ½ cup sugar, 5 tbsp unsalted butter, 3 large eggs, 2¼ tsp active dry yeast
Add 4½ cups flour and salt to the mixture. Mix on low just until the dough starts to come together. Use your hands to remove the dough from the paddle attachment. Remove the paddle and replace it with the dough hook. Mix (knead) on medium for 6 to 7 minutes until the dough is soft and is starting to pull away from the bowl, adding another ¼ to ½ cup of flour as needed.
5 cups all-purpose flour, 1 tsp salt
Remove the dough from the bowl and smooth it into a ball with your hands. The dough will be quite sticky. This is normal. Spray a large bowl with cooking spray and place the dough in the bowl. Turn the dough over to coat with oil all over. If desired, brush melted butter over the top of the dough. Cover with plastic wrap and place in a warm, non-drafty area until it has doubled in size, usually about 2 hours. Or, place in the refrigerator for overnight or up to 12 hours.
On a floured work surface, roll out the dough to ½-inch thickness.
Using a 2½-inch cookie cutter, an inverted metal can, or an inverted glass, cut rounds from the dough. Re-roll the scraps, and cut more rounds.
Place each round on a parchment paper-lined baking sheet, about 1 inch apart from each other, cover with a damp towel, and let rise at room temperature until nearly doubled in size, about 45 minutes to 2 hours.
Preheat oven to 350°F.
Using your thumb, press 1 large indention into the center of each round, leaving a 1/2-inch wide rim. Spoon about 1 tablespoon topping into the indentions. Liberally sprinkle the streusel over the tops of the kolaches. Brush the melted butter all over the edges of the kolaches.
2 tbsp unsalted butter
Bake until golden brown, about 15 to 25 minutes (depending on the size of the kolaches).
Transfer to a wire rack and let cool