My Mother's Butterscotch Refrigerator Cookies

These are also called Icebox Cookies. I loved watching my mother make them every Christmas. Her recipe is a little extra! 

1 cup butter  1 cup white sugar   1 cup brown sugar  2 eggs, separated   5 cups flour  1 teaspoon baking soda   1/2 teaspoon salt   1/2 cup thick sour cream   1+1/2 teaspoon vanilla  1 cup chopped nuts (pecans or walnuts) 

Cream together butter and sugars.  Beat egg yolks, add in and mix til smooth.  Sift together flour, baking soda, and salt  Add sour cream, vanilla and dry ingredients to sugar-egg-butter mixture. Mix well. This may take some time as the the mixture will become very thick.  Blend in stiffly beaten egg whites and nuts. Shape into loaves, 2-inches square.