For the dough:
In a large bowl combine the flour, yeast and salt; mix together.
Add in the sour cream and softened butter. Use your hands and work the mixture into a dough.
Knead for about 5 minutes or until the dough is no longer sticky.
Place the dough in a bowl, cover and refrigerate for 3 hours.
For the filing:
Mix together the flour, sugars, softened butter in a medium bowl.
Add in the vanilla and ground walnuts.
Preheat the oven to 350 degrees.
Remove the dough from the refrigerator.
Form a ball and sprinkle your work surface with flour.
Roll out the dough into a large rectangle. The dough should be thin but not transparent
Spread the filling on the dough keeping it about ½ inch away from the edges.
Starting on the long side, start rolling up the dough, making sure the filling is evenly distributed.
Place the dough seam side down and press on it slightly to flatten the top.
Cut the dough into 12 even pieces and place on an un-greased cookie sheet.
Brush the tops of all the pieces with the beaten egg.
Bake for 30 minutes or until golden brown.
Remove the pastries from the cookie sheet and allow to cool on a wire rack.
The nazook will keep up to a week in an airtight container.