In a large bowl, stir together the warm milk, sugar and yeast. Let it sit for 5-10 minutes, until foamy.
Whisk in the melted butter, sour cream and salt. Add the flour 1 cup at a time, stirring until a soft dough forms.The sour cream replaces the egg yolks found in classic kolache dough and keeps these buns rich, soft and slightly sweet.
Knead the dough on a lightly floured surface for 6-8 minutes, until smooth and slightly tacky. Place it in a greased bowl, cover and let it rise in a warm spot for about 1 hour, until doubled.
While the dough rises, mix the flour and sugar for the streusel in a small bowl. Rub in the cold butter with your fingertips until crumbly, then chill until needed.
Punch down the dough, divide it into 12 equal pieces and roll each into a smooth ball. Place them 2 inches apart on a parchment-lined baking sheet, flatten slightly, cover and let rise for 30-40 minutes. Preheat the oven to 375°F while they rise.
Press a deep well into the center of each bun with two fingers or the bottom of a small glass, leaving a 1/2-inch rim. Brush the rims with some of the melted butter, spoon 1 tablespoon of preserves into each well and sprinkle the streusel over the top.
Bake for 15-18 minutes, until golden brown. Brush the edges with the remaining melted butter right out of the oven and let the kolaches cool for 10 minutes before serving.