Pour warm milk (between 110 and 115F) into large bowl and sprinkle yeast on top. Let yeast dissolve and add sugar. Whisk in the beaten eggs, melted butter and sour cream until smooth batter forms. Gradually, fold in ½ cup of flour at a time until soft dough forms. Invert dough onto clean, lightly floured surface and knead for several minutes until dough is uniform and gains elasticity. Divide dough into 4 equal sized pieces. Do not let dough rise - make the cookies right away.
Preheat the oven to 350F. Working with one piece of dough at a time, roll out the dough into a 10 to 12-inch circle. Then using very sharp knife or pizza cutter, slice the circle into 8 pieces like you would a pie.
Place 1/2 to 1 teaspoon fruit preserves on the outer edge of each slice and fold the dough over the fruit preserves. Roll the cookies to form crescent shape and place onto well-greased or parchment-lined cookie sheet. Let the cookies rise, doubling in size before baking.
Bake the cookies for approximately 12 minutes, or until lightly golden brown on edges and top. Remove the cookies from oven and using spatula, remove the cookies from pan. Place them onto a cooling rack to cool.
If desired, place the hot cookies directly into a bowl of powdered sugar and coat them completely. Gently remove them from the powdered sugar using tongs and onto cooling racks. Once the cookies have cooled, dust them lightly with powdered sugar again. Store the cookies in airtight container.