In a large bowl, using a pastry blender, blend the butter into the flour creating a coarse, crumb-like consistency.
In a separate bowl, mix the egg yolks and sour cream. Add to flour mixture. Blend and knead lightly until mixture is like pie dough. Shape into small, walnut-sized balls, cover, and refrigerate overnight (makes about 52-56 balls).
Place egg whites in an airtight container and refrigerate.
The next day, beat the egg whites until stiff. Add powdered sugar and beat well. Stir in nuts and refrigerate.
To a medium bowl, add 2 tablespoons each of all-purpose flour and sugar. Take half the balls out of the refrigerator and place in flour/sugar mixture. Keep the rest chilled.
Roll balls out on a small amount of flour/sugar mixture into a thin circle. Place a heaping teaspoon of filling on each and roll up into a crescent shape. Pinch the ends tightly. Place seam-side down on a greased cookie sheet.
Bake at 350 degrees Fahrenheit for 12 to 15 minutes. Cool slightly and dip in a bowl of confectioners' sugar. Store the kiflies in a cool place.
You can add a teaspoon of vanilla to the filling.
Make sure to roll your dough very thin, or it will puff up and crack.
Some filling might come out. No worries, there will still be enough left in the cookies.
For nut allergies, replace the walnut filling with your favorite jam or pastry fruit filling.
Bake the cookies distributed between two large cookie sheets on convection setting or bake in batches.
Freeze baked and cooled cookies but not covered in sugar. Lay them on the counter for 2 hours to thaw and then dust with powdered sugar if desired.
Please keep in mind that nutritional information is a rough estimate and can vary significantly based on products used.