In a small bowl, dissolve the sugar in warm milk. Sprinkle the yeast on top and let it sit for about 5-10 minutes until foamy.
In a large mixing bowl, combine the flour and salt. Cut in the butter until the mixture resembles coarse crumbs.
Make a well in the center and add the yeast mixture and egg. Mix until a dough forms.
Turn the dough onto a floured surface and knead for about 5 minutes, or until smooth. Place the dough in a lightly oiled bowl, cover with a damp cloth, and let it rise in a warm place for about 1 hour, or until doubled in size.
Once the dough has risen, turn it onto a floured surface and roll it out to about 1/2 inch thickness. Cut into rounds using a cookie cutter or glass.
Place the rounds on a baking sheet lined with parchment paper. Make a small indentation in the center of each round using your finger or a small spoon.
Spoon a small amount of fruit preserves or cheese filling into each indentation.
Cover the filled dough rounds with a cloth and let them rise for about 30 minutes.
Preheat the oven to 375°F (190°C). Brush the edges of the dough with the beaten egg.
Bake the koláče for 12-15 minutes, or until golden brown.
Allow the koláče to cool slightly on a wire rack. Dust with powdered sugar if desired.
Enjoy your meal!