• Combine the yeast, water, and sugar in a small bowl. Prepare a setting.
• Combine the flour, salt, and cold butter in a large bowl.
• Add the egg yolks (save the whites! ), sour cream, and yeast mixer to the well at the center. A frothy dome on top indicates that your yeast has "bloomed" properly. The temperature of the water is 100% responsible for this.
• Form a dough ball by mixing these ingredients together.
• Clean the bowl after removing the dough from it. Prepare a clean bowl by greasing it and placing the dough back inches. 
• Cover the bowl with a tea towel and place it in a warm place while you make the filling. It rises just fine when I microwave it or put it in the oven.

To Make The Filling:

• Using a heavy bottom saucepan, heat the filling ingredients to a boil over medium heat. Make sure you stir constantly. Do not leave it unattended for more than 20 minutes, or it will scorch.
• Pour into a clean bowl and chill until completely cool. Every 15 minutes or so, I put it in the freezer and stir it.
• Divide the dough into four balls once the filling has cooled.
• Once they have been rolled out, cut them into 12"-15" squares on a lightly floured surface. In order to save space, stack them together while you fill them.
• On cookie sheets, roll them loosely into logs and fill with poppy seed filling about 1/2" from the edge.
• After 20 minutes, allow them to rise a little.
• Heat the oven to 425 degrees Fahrenheit.
• With the beaten egg white, rolls are made.
• For 25-30 minutes, bake.
• Cut into 1/2-inch slices when cooled on wire racks.