Warm the milk to 105-110°F. Stir in the yeast and a small pinch of the sugar, then let the mixture stand for 5-10 minutes until foamy.

Mix the foamy yeast mixture with the remaining sugar, melted butter, eggs, and salt. Add 4 cups of flour gradually until a soft dough forms.

Add more flour only if the dough is excessively sticky, using 1 tablespoon at a time. Knead by hand for about 8 minutes or with a dough hook for 5-6 minutes, until smooth and elastic.

Place the dough in a lightly greased bowl, cover, and let it rise in a warm, draft-free place for 60-90 minutes, or until doubled.

Beat the softened cream cheese, sugar, egg yolk, vanilla, and salt until completely smooth. Set the filling aside while the dough finishes rising.

Divide the risen dough into 24 equal pieces. Shape each piece into a ball and arrange the balls on parchment-lined baking sheets.

Flatten each dough ball slightly. Press a deep indentation into the center with your thumb or the back of a rounded measuring spoon, taking care not to press all the way through.

Spoon approximately 1 tablespoon of cream cheese filling into each indentation. Add about ½ teaspoon of fruit preserves to each pastry if desired.

Cover the shaped kolaches loosely and let them rise again for 20-30 minutes. Meanwhile, heat the oven to 350°F.

Bake the kolaches for 12-16 minutes, until the edges are lightly golden and the cream cheese centers look set.

Cool the pastries on the baking sheet for several minutes before transferring them to a wire rack. The filling becomes firmer as it cools.

Dust the cooled kolaches with powdered sugar if desired and serve slightly warm or at room temperature.